[Fizz] Simplify ReplayNode data structure (#27395)

The key is that instead of storing different tags of resumable points,
we just store if a replay node has any resumable slots and if that's at
the root `number` or if it has resumable slots by index.

This is a simpler and more compact format because we don't have to
separate the three Resume forms.

This helps deal with Postpone in fallbacks because it doesn't just
double all the cases.

Author: sebmarkbage