fix: skip calling respond for server-side fetch on prerendered pages

[aloisklink]

Server-side fetch() (e.g. when isSubRequest: true) currently has a few bugs when called on a prerendered route.

1. When a non-prerendered route also matches the same URL as the prerendered route, the non-prerendered routes are taking precedence (e.g. if export const prerender = 'auto'). Fixes Global slug overrides prerendered route when accessed from another route with event.fetch, but ok from client? #12739.
2. Prerendered routes still have hooks.server.js run on them, but only when isSubRequest: true

I've fixed both of these issues by adding a prerendered_routes: Set<string> to the manifest. Then, we instantly call a real HTTP fetch() if the path matches, just like when calling server-side fetch() on assets (when the adapter doesn't support reading assets from the file-system).

During dev/build, there are no prerendered_routes, so it's left empty in the manifest (although that does mean the manifest-full.js also excludes it!)

Potential issues

• According to the docs for prerendered.paths (which I'm using to set prerendered_routes), it only has the routes without a trailing slash. Would this be an issue? If so, the code might get a lot more complicated!

  kit/packages/kit/src/types/private.d.ts

  Lines 189 to 190 in 873a09e

  	/** An array of prerendered paths (without trailing slashes, regardless of the trailingSlash config) */
  	paths: string[];

• In the future, we might want to make a new entry like prerendered_redirects, so that we can follow the redirect chain without having to make a real HTTP request, but I think that should be fine if this is under the private _ field in the manifest.js file.

---

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  • Fixes Global slug overrides prerendered route when accessed from another route with event.fetch, but ok from client? #12739. I'm not super sure if this implementation is the best way to fix it, though!
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