fix: detect build-emitted assets in prerenderer fetch

[JelmerBot]

This PR solves #12200.

Assets emitted from a vite-plugin cannot be loaded by the pre-renderer's fetch implementation because they are not listed in the server manifest. This PR detects files the server's asset location (server/_app/immutable/assets) and add them the manifest's asset list:
https://github.com/JelmerBot/kit/blob/0fd65d309f029b36de41e28a3e8946d893e9385f/packages/kit/src/core/generate_manifest/index.js#L83C2-L90C3

The pre-renderer's read is updated to detect these asset links:
https://github.com/JelmerBot/kit/blob/0fd65d309f029b36de41e28a3e8946d893e9385f/packages/kit/src/core/postbuild/prerender.js#L214C5-L217C6

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[changeset-bot]

🦋 Changeset detected

Latest commit: 5dfed36

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

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-12201-svelte.vercel.app/

[Rich-Harris]

very late, but thank you!