Add debug logging to default cache handler and `"use cache"` wrapper (#77827)
When the env variable `NEXT_PRIVATE_DEBUG_CACHE` is defined (see https://nextjs.org/docs/app/building-your-application/data-fetching/incremental-static-regeneration#verifying-correct-production-behavior), the default cache handler and the `"use cache"` wrapper will now emit debug logs.
Note: This variable can already be used for `"use cache"` debug logging when deploying to Vercel.