Summary:
`SynchronousEventBeat` and `AsynchronousEventBeat` are a cross-platform re-implementation of run loop related parts of `MainRunLoopEventBeat` and `RuntimeEventBeat` (iOS specific classes for now). In the future, they will replace iOS- and Android-specifc event beat classes.
Changelog: [Internal] Fabric-specific internal change.
Reviewed By: sammy-SC
Differential Revision: D21341996
fbshipit-source-id: 8eda9a5df537cd666b7728e32212a8bb5ddb3ab7
