feat: preserve existing unknown .github/DEVELOPMENT.md sections (#1173)

## PR Checklist

- [x] Addresses an existing open issue: fixes #1167
- [x] That issue was marked as [`status: accepting
prs`](https://github.com/JoshuaKGoldberg/create-typescript-app/issues?q=is%3Aopen+is%3Aissue+label%3A%22status%3A+accepting+prs%22)
- [x] Steps in
[CONTRIBUTING.md](https://github.com/JoshuaKGoldberg/create-typescript-app/blob/main/.github/CONTRIBUTING.md)
were taken

## Overview

Now, when `.github/DEVELOPMENT.md` exists, any `##` that doesn't match
the new headings will be preserved and moved to the bottom of the file.

Note that this doesn't show up nicely in the migration test end-to-end
snapshots because `updateLocalFiles` manually removes the `## Setup
Scripts` sections.

Author: JoshuaKGoldberg

script/__snapshots__/migrate-test-e2e.js.snap

@@ -174,8 +174,10 @@ exports[`expected file changes > cspell.json 1`] = `
  		"mtfoley",
  		"npmignore",
 @@ ... @@
+ 		"packagejson",
  		"quickstart",
  		"tada",
++		"templating",
  		"tsup",
 -		"vitest"
 +		"vitest",

src/steps/writing/creation/dotGitHub/createDevelopment.test.ts src/steps/writing/creation/dotGitHub/createDevelopment/index.test.ts

@@ -1,7 +1,15 @@
-import { describe, expect, it } from "vitest";
+import { describe, expect, it, vi } from "vitest";
 
-import { Options } from "../../../../shared/types.js";
-import { createDevelopment } from "./createDevelopment.js";
+import { Options } from "../../../../../shared/types.js";
+import { createDevelopment } from "./index.js";
+
+const mockReadFileSafe = vi.fn();
+
+vi.mock("../../../../../shared/readFileSafe.js", () => ({
+	get readFileSafe() {
+		return mockReadFileSafe;
+	},
+}));
 
 const options = {
 	access: "public",
@@ -21,8 +29,10 @@ const options = {
 } satisfies Options;
 
 describe("createDevelopment", () => {
-	it("creates a file with extra options turned on when options disable them", () => {
-		const actual = createDevelopment({
+	it("creates a file with extra options turned on when options disable them", async () => {
+		mockReadFileSafe.mockResolvedValue("");
+
+		const actual = await createDevelopment({
 			...options,
 			excludeLintKnip: false,
 			excludeLintMd: false,
@@ -77,7 +87,7 @@ describe("createDevelopment", () => {
 
 			- \`pnpm lint\` ([ESLint](https://eslint.org) with [typescript-eslint](https://typescript-eslint.io)): Lints JavaScript and TypeScript source files
 			- \`pnpm lint:knip\` ([knip](https://github.com/webpro/knip)): Detects unused files, dependencies, and code exports
-			- \`pnpm lint:md\` ([Markdownlint](https://github.com/DavidAnson/markdownlint)): Checks Markdown source files
+			- \`pnpm lint:md\` ([Markdownlint](https://github.com/DavidAnson/markdownlint): Checks Markdown source files
 			- \`pnpm lint:package-json\` ([npm-package-json-lint](https://npmpackagejsonlint.org/)): Lints the \`package.json\` file
 			- \`pnpm lint:packages\` ([pnpm dedupe --check](https://pnpm.io/cli/dedupe)): Checks for unnecessarily duplicated packages in the \`pnpm-lock.yml\` file
 			- \`pnpm lint:spelling\` ([cspell](https://cspell.org)): Spell checks across all source files
@@ -134,8 +144,10 @@ describe("createDevelopment", () => {
 		`);
 	});
 
-	it("creates a file with extra options turned off when options enable them", () => {
-		const actual = createDevelopment({
+	it("creates a file with extra options turned off when options enable them", async () => {
+		mockReadFileSafe.mockResolvedValue("");
+
+		const actual = await createDevelopment({
 			...options,
 			excludeLintKnip: true,
 			excludeLintMd: true,
@@ -192,7 +204,7 @@ describe("createDevelopment", () => {
 
 			## Linting
 
-			[ESLint](https://eslint.org) is used with with [typescript-eslint](https://typescript-eslint.io) to lint JavaScript and TypeScript source files.
+			[ESLint](https://eslint.org) is used with with [typescript-eslint](https://typescript-eslint.io)) to lint JavaScript and TypeScript source files.
 			You can run it locally on the command-line:
 
 			\`\`\`shell
@@ -248,4 +260,136 @@ describe("createDevelopment", () => {
 			"
 		`);
 	});
+
+	it("preserves existing sections when they don't match the new sections", async () => {
+		mockReadFileSafe.mockResolvedValue(`## Existing One
+
+Abc 123.
+
+## Building
+
+Will be removed.
+
+## Tests
+
+Will be removed.
+
+## Existing Two
+
+Def 456.
+`);
+
+		const actual = await createDevelopment({
+			...options,
+			excludeLintKnip: false,
+			excludeLintMd: false,
+			excludeLintPackageJson: false,
+			excludeLintPackages: false,
+			excludeLintSpelling: false,
+		});
+
+		expect(actual).toMatchInlineSnapshot(`
+			"# Development
+
+			After [forking the repo from GitHub](https://help.github.com/articles/fork-a-repo) and [installing pnpm](https://pnpm.io/installation):
+
+			\`\`\`shell
+			git clone https://github.com/<your-name-here>/test-repository
+			cd test-repository
+			pnpm install
+			\`\`\`
+
+			> This repository includes a list of suggested VS Code extensions.
+			> It's a good idea to use [VS Code](https://code.visualstudio.com) and accept its suggestion to install them, as they'll help with development.
+
+			## Building
+
+			Will be removed.
+
+			## Formatting
+
+			[Prettier](https://prettier.io) is used to format code.
+			It should be applied automatically when you save files in VS Code or make a Git commit.
+
+			To manually reformat all files, you can run:
+
+			\`\`\`shell
+			pnpm format --write
+			\`\`\`
+
+			## Linting
+
+			This package includes several forms of linting to enforce consistent code quality and styling.
+			Each should be shown in VS Code, and can be run manually on the command-line:
+
+			- \`pnpm lint\` ([ESLint](https://eslint.org) with [typescript-eslint](https://typescript-eslint.io)): Lints JavaScript and TypeScript source files
+			- \`pnpm lint:knip\` ([knip](https://github.com/webpro/knip)): Detects unused files, dependencies, and code exports
+			- \`pnpm lint:md\` ([Markdownlint](https://github.com/DavidAnson/markdownlint): Checks Markdown source files
+			- \`pnpm lint:package-json\` ([npm-package-json-lint](https://npmpackagejsonlint.org/)): Lints the \`package.json\` file
+			- \`pnpm lint:packages\` ([pnpm dedupe --check](https://pnpm.io/cli/dedupe)): Checks for unnecessarily duplicated packages in the \`pnpm-lock.yml\` file
+			- \`pnpm lint:spelling\` ([cspell](https://cspell.org)): Spell checks across all source files
+
+			Read the individual documentation for each linter to understand how it can be configured and used best.
+
+			For example, ESLint can be run with \`--fix\` to auto-fix some lint rule complaints:
+
+			\`\`\`shell
+			pnpm run lint --fix
+			\`\`\`
+
+			Note that you'll likely need to run \`pnpm build\` before \`pnpm lint\` so that lint rules which check the file system can pick up on any built files.
+
+			## Testing
+
+			[Vitest](https://vitest.dev) is used for tests.
+			You can run it locally on the command-line:
+
+			\`\`\`shell
+			pnpm run test
+			\`\`\`
+
+			Add the \`--coverage\` flag to compute test coverage and place reports in the \`coverage/\` directory:
+
+			\`\`\`shell
+			pnpm run test --coverage
+			\`\`\`
+
+			Note that [console-fail-test](https://github.com/JoshuaKGoldberg/console-fail-test) is enabled for all test runs.
+			Calls to \`console.log\`, \`console.warn\`, and other console methods will cause a test to fail.
+
+			### Debugging Tests
+
+			This repository includes a [VS Code launch configuration](https://code.visualstudio.com/docs/editor/debugging) for debugging unit tests.
+			To launch it, open a test file, then run _Debug Current Test File_ from the VS Code Debug panel (or press F5).
+
+			## Type Checking
+
+			You should be able to see suggestions from [TypeScript](https://typescriptlang.org) in your editor for all open files.
+
+			However, it can be useful to run the TypeScript command-line (\`tsc\`) to type check all files in \`src/\`:
+
+			\`\`\`shell
+			pnpm tsc
+			\`\`\`
+
+			Add \`--watch\` to keep the type checker running in a watch mode that updates the display as you save files:
+
+			\`\`\`shell
+			pnpm tsc --watch
+			\`\`\`
+
+			## Existing One
+
+			Abc 123.
+
+			## Tests
+
+			Will be removed.
+
+			## Existing Two
+
+			Def 456.
+			"
+		`);
+	});
 });