Enable synchronous state update dispatching behind a flag (#43580)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/43580

Changelog: [internal]

This introduces a new feature flag to commit state updates synchronously from the UI thread (generally) instead of dispatching them to the JS thread to be processed there. We can do this now because we introduced a UI consistency mechanism in D55024832 to JS would see a consistent revision during the execution of a specific task.

Reviewed By: sammy-SC

Differential Revision: D55083029

fbshipit-source-id: 8aa84ddaee383f098252fa679cfb07012ba29bf8