dillon1000/react

Refactor React Server entrypoint to not depend on the client one (#27940)

This refactors the Server Components entrypoint for the `react` package
(ReactServer.js) so that it doesn't depend on the client entrypoint
(React.js). I also renamed React.js to ReactClient.js to make the
separation clearer.

This structure will make it easier to add client-only and server-only
features.