fix: write server files to _worker.js/ directory

[teemingc]

fixes #13300

This PR changes the written _worker.js file to a directory, allowing us to include the server files in the .svelte-kit/cloudflare build output without making them public. It was not possible to include the server files before because all the files in the build output are made public, but there is an undocumented feature in Wrangler that allows us to store all server files in a _worker.js/ directory to be bundled. The result is that all build artefacts are once again stored in a single directory instead of linking server files relatively to the Vite output folder.

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Latest commit: 57abafb

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

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Name	Type
@sveltejs/adapter-cloudflare	Patch

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