re-introduce partial fallback -> route upgrading (#89063)
In https://github.com/vercel/next.js/pull/69282, we landed support for
Partial Fallback Prerendering. The intention behind this feature was to
be able to upgrade from a partial fallback (essentially the UI produced
when `await params` is dynamic, ie the param was not part of
`generateStaticParams` at build time) to a route shell (when the param
values are filled in, and thus we can prerender more of the page).
It was reverted in https://github.com/vercel/next.js/pull/79258 because
we didn't add first-class support to this in the build output API and
the necessary infrastructure changes to perform similar behavior that
this PR does to trigger a background revalidation with params filled in.
This re-lands the feature in Next.js now that upstream support landed
(https://github.com/vercel/vercel/pull/14703,
https://github.com/vercel/vercel/pull/15338, and Vercel infra changes
which are rolling out)
