Adds a new field `prefetchRsc` to CacheNode that will be used by the PPR
implementation. It represents a static version of the segment that can
be showed immediately, and may or may not contain dynamic holes. It's
prefetched before a navigation occurs. During rendering, we will choose
whether to render `rsc` or `prefetchRsc` with `useDeferredValue`.
As with the `rsc` field, a value of `null` means no value was provided.
In this case, the LayoutRouter will go straight to rendering the `rsc`
value; if that one is also missing, it will suspend and trigger a lazy
fetch.
The non-PPR implementation will never set this value.
This PR adds the field to the CacheNode type but doesn't implement any
of the behavior yet. Mostly this involves updating the router reducer
unit tests.
Closes NEXT-1855
5adacb6912