dillon1000/react

Add LegacyHidden to server renderer (#18919)

* Add LegacyHidden to server renderer

When the tree is hidden, the server renderer renders nothing. The
contents will be completely client rendered.

When the tree is visible it acts like a fragment.

The future streaming server renderer may want to pre-render these trees
and send them down in chunks, as with Suspense boundaries.

* Force client render, even at Offscreen pri