[ppr] Route Cardinality Updates (#78476)
This enables a new set of cardinality changes for pages with PPR and
Dynamic IO enabled on it. When a set of more specific routes are
generated, we no longer error when the static shell is empty and instead
infer that as a sign that the user doesn't want to provide a route
fallback. If the user does in fact place suspense boundaries above
params access, the fallback shell will not be empty and will be used.
Given the following application structure:
```
/[lang]/layout.tsx
/[lang]/[teamSlug]/page.tsx
```
Where we have the following returned for `generateStaticProps`: `[{
lang: "en", teamSlug: "nextjs"}]`, we'd have the following rendered:
```
/[lang]/[teamSlug] <- empty shell won't cause error
/en/[teamSlug] <- empty shell won't cause error
/en/nextjs <- empty shell will cause error
```
If `/[lang]/[teamSlug]` generated an empty shell, then a request to
`/fr/nextjs` wouldn't use the `/[lang]/[teamSlug]` fallback shell and
would instead generate a new blocking route shell. Otherwise if it
didn't generate an empty shell, it would use the fallback shell that's
outputted.
If `/en/[teamSlug]` generated an empty shell, then a request to
`/en/react` wouldn't use the `/en/[teamSlug]` fallback shell and would
instead generate a new blocking route shell. Otherwise if it didn't
generate an empty shell, it would use the fallback shell that's
outputted.
