[Fix] Add prerendered assets to Vercel config

[pawelblaszczyk5]

Hello!
Solves #8331

Before prerendered data.json or other non-html pages weren't included in Vercel config. It correctly adds them to it.

I monkey patched the change to my private project and it correctly worked. While fixing I found that it's also applicable to adapter Vercel. However, I'm not familiar with the codebase and the adapters system, so I'm not sure about the correctness of my approach. I'm also not sure about the test for the change but can provide one if given some advice and it's needed 😀

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🦋 Changeset detected

Latest commit: 048dcfe

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

This PR includes changesets to release 2 packages

Name	Type
@sveltejs/adapter-vercel	Patch
@sveltejs/adapter-static	Patch

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[Rich-Harris]

Thank you. adapter-vercel was totally fine (except for an unrelated bug — it was adding immutable cache headers to non-immutable assets), so rather than changing it, I've copied the logic from there into adapter-static. Using overrides is preferable to using rewrite routes.

(This does result in some duplication, but it's at least explicit. Probably not worth creating a new package that they both depend on in order to reduce that duplication.)

[pawelblaszczyk5]

Thanks a lot, I wasn't sure, because I was surprised by the totally different code between these two. Good idea to unify them 😊