Initial shim of useSyncExternalStore

This sets up an initial shim implementation of useSyncExternalStore,
via the use-sync-external-store package. It's designed to mimic the
behavior of the built-in API, but is backwards compatible to any version
of React that supports hooks.

I have not yet implemented the built-in API, but once it exists, the
use-sync-external-store package will always prefer that one. Library
authors can depend on the shim and trust that their users get the
correct implementation.

See reactwg/react-18#86 for background
on the API.

The tests I've added here are designed to run against both the shim and
built-in implementation, using our variant test flag feature. Tests that
only apply to concurrent roots will live in a separate suite.