fix: only generate prefetch rsc for ppr enabled routes (#66395)
This adds details for every ISR cache request if the page being
requested supports PPR. If it does, it'll attempt to load the
`.prefetch.rsc` payload instead of the `.rsc` payload. This corrects a
bug that was present in deployed environments.
This additionally refactors the `isAppPPREnabled` out of most of the
application, as it's only used to determine if we should add to the
`prefetchDataRoute` fields in the `prerender-manifest.json`. To support
loading the prefetch file or not, we pass the `isRoutePPREnabled`
through with the cache get/set operations instead.
x-slack-ref:
https://vercel.slack.com/archives/C075MSFK9ML/p1717094328986429
