feat: added partial shell generation using root params (#73816)
This enables the generation of partial shells when using with partial prerendering, aided by the new `rootParams()` API. Essentially, when your application only returns partial routes (where not all the route parameters are known), Next.js will now build route shells for these pages. We call these shells, fallback shells. They represent the partial state of the page that once served to the user, will provide a more complete loading experience as fast as possible.
We'll also take every permutation of the provided root params and generate a shell just for them to ensure that any future calls to those routes will be able to use the more specific fallback shell that's generated rather than having to rely on a blank shell by default.
