Add flight specific entry point for react package (#20304)

This configures the Flight fixture to use the "react-server" environment.

This allows the package.json exports field to specify a different resolution
in this environment.

I use this in the "react" package to resolve to a new bundle that excludes
the Hooks that aren't relevant in this environment like useState and useEffect.

This allows us to error early if these names are imported. If we actually
published ESM, it would we a static error. Now it's a runtime error.

You can test this by importing useState in Container.js which is used
by the client and server.