fix: allow setting files binding for <input type="file" /> (Svelte 4)

[teemingc]

fixes #8316

This PR allows setting the files binding for file inputs to allow for common operations such as setting the files to a drag and drop file, or clearing the file input selection. Currently, this is not possible because bind:files is read-only in Svelte. The redundant alternative is to bind the input element instead and set the files property there.

<script>
  let files;
</script>

<input type="file" bind:files />

<!-- Reset file input -->
<button on:click={() => files = new DataTransfer().files}>Reset</button>

<!-- Set file input as the dragged file -->
<svelte:window
	on:drop|preventDefault={(event) => {
		const { dataTransfer } = event;
		if (!dataTransfer) return;

		files = dataTransfer.files;
	}}
	on:dragover|preventDefault
/>

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[changeset-bot]

🦋 Changeset detected

Latest commit: 1b2dba6

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

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svelte	Patch

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[benmccann]

@s3812497 it looks like this PR has a merge conflict

[teemingc]

resolved the merge conflict

[dummdidumm]

#10032 revealed that this would actually be a breaking change, therefore closing