feat: expose submitter in SubmitFunction (#9425)

closes #9414

Author: msonnberger

Diff for: .changeset/lemon-mayflies-design.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': minor
+---
+
+feat: expose submitter in use:enhance SubmitFunction

Diff for: documentation/docs/20-core-concepts/30-form-actions.md

@@ -344,11 +344,12 @@ To customise the behaviour, you can provide a `SubmitFunction` that runs immedia
 ```svelte
 <form
 	method="POST"
-	use:enhance={({ form, data, action, cancel }) => {
+	use:enhance={({ form, data, action, cancel, submitter }) => {
 		// `form` is the `<form>` element
 		// `data` is its `FormData` object
 		// `action` is the URL to which the form is posted
 		// `cancel()` will prevent the submission
+		// `submitter` is the `HTMLElement` that caused the form to be submitted
 
 		return async ({ result, update }) => {
 			// `result` is an `ActionResult` object

Diff for: packages/kit/src/runtime/app/forms.js

@@ -91,7 +91,8 @@ export function enhance(form, submit = () => {}) {
 				cancel,
 				controller,
 				data,
-				form
+				form,
+				submitter: event.submitter
 			})) ?? fallback_callback;
 		if (cancelled) return;
 

Diff for: packages/kit/types/ambient.d.ts

@@ -84,6 +84,7 @@ declare module '$app/forms' {
 		form: HTMLFormElement;
 		controller: AbortController;
 		cancel(): void;
+		submitter: HTMLElement | null;
 	}) => MaybePromise<
 		| void
 		| ((opts: {