Support shell prerendering of dynamic routes

[andriygm]

For a pre-rendered SPA with the following routes:

/
/page/<slug>/
/page/<slug>/settings

TanStack Start can currently only prerender the / route unless you provide explicit pages to prerender either in the config or as links within other pages and allow the crawler to find (as explained here https://tanstack.com/start/latest/docs/framework/react/guide/static-prerendering#automatic-static-route-discovery). We can improve UX/perceived load times by generating prerendered shells for these pages which could include route-specific skeletons/layout (slug-specific content would still need to come in after hydration of course).