feat: preserve existing unknown .github/DEVELOPMENT.md sections

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 🚀 Feature: In --mode migrate, preserve existing .github/DEVELOPMENT.md body contents #1167
• That issue was marked as status: accepting prs
• Steps in 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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a21348a) 94.69% compared to head (50a4ed1) 94.78%.
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1173      +/-   ##
==========================================
+ Coverage   94.69%   94.78%   +0.09%     
==========================================
  Files         102      104       +2     
  Lines        5689     5775      +86     
  Branches      468      482      +14     
==========================================
+ Hits         5387     5474      +87     
+ Misses        301      300       -1     
  Partials        1        1              

Flag	Coverage Δ
create	70.34% <76.11%> (+0.08%)	⬆️
initialize	42.34% <23.38%> (-0.14%)	⬇️
migrate	70.24% <90.04%> (+0.42%)	⬆️
unit	74.42% <92.53%> (+0.59%)	⬆️

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[github-actions]

🎉 This is included in version v1.54.0 🎉

The release is available on:

• GitHub releases
• npm package (@latest dist-tag)

Cheers! 📦🚀