dillon1000/react

Make ART Concurrent if Legacy Mode is disabled (#28662)

Pulling this out of #28657.

This runs react-art in concurrent mode if disableLegacyMode is true.
Effectively this means that the OSS version will be in concurrent mode
and the `.modern.js` version for Meta will be in concurrent mode, once
the flag flips for modern, but the `.classic.js` version for Meta will
be in legacy mode.

Updates flowing in from above flush synchronously so that they commit as
a unit. This also ensures that refs are resolved before the parent life
cycles. setStates deep in the tree will now be batched using "discrete"
priority but should still happen same task.