dillon1000/react

Add SuspenseList Component (#15902)

* Add SuspenseList component type

* Push SuspenseContext for SuspenseList

* Force Suspense boundaries into their fallback state

In the "together" mode, we do a second render pass that forces the
fallbacks to stay in place, if not all can unsuspend at once.

* Add test

* Transfer thennables to the SuspenseList

This way, we end up retrying the SuspenseList in case the nested boundary
that just suspended doesn't actually get mounted with this set of
thennables. This happens when the second pass renders the fallback
directly without first attempting to render the content.

* Add warning for unsupported displayOrder

* Add tests for nested sibling boundaries and nested lists

* Fix nested SuspenseList forwarding thennables

* Rename displayOrder to revealOrder

Display order has some "display list" connotations making it sound like
a z-index thing.

Reveal indicates that this isn't really about when something gets rendered
or is ready to be rendered. It's about when content that is already there
gets to be revealed.

* Add test for avoided boundaries

* Make SuspenseList a noop in legacy mode

* Use an explicit suspense list state object

This will be used for more things in the directional case.