dillon1000/react

Schedule prerender after something suspends (#30800)

Adds the concept of a "prerender". These special renders are spawned
whenever something suspends (and we're not already prerendering).

The purpose is to move speculative rendering work into a separate phase
that does not block the UI from updating. For example, during a
transition, if something suspends, we should not speculatively prerender
siblings that will be replaced by a fallback in the UI until *after* the
fallback has been shown to the user.