dillon1000/react

[Re-land] Make prerendering always non-blocking (#31268)

Follows https://github.com/facebook/react/pull/31238

___

This is a partial re-land of
https://github.com/facebook/react/pull/31056. We saw breakages surface
after the original land and had to revert. Now that they've been fixed,
let's try this again. This time we'll split up the commits to give us
more control of testing and rollout internally.

Original PR: https://github.com/facebook/react/pull/31056
Original Commit:
https://github.com/facebook/react/pull/31056/commits/4c71025d8d1bd46344ad793e7ed3049d24f7395a
Revert PR: https://github.com/facebook/react/pull/31080

Commit description:

> When a synchronous update suspends, and we prerender the siblings, the
prerendering should be non-blocking so that we can immediately restart
once the data arrives.
>
> This happens automatically when there's a Suspense boundary, because
we immediately commit the boundary and then proceed to a Retry render,
which are always concurrent. When there's not a Suspense boundary, there
is no Retry, so we need to take care to switch from the synchronous work
loop to the concurrent one, to enable time slicing.

Co-authored-by: Andrew Clark <[email protected]>