Summary:
A follow up to D38708718 (https://github.com/facebook/react-native/commit/403fea25f65a38f4b4d8e0edcf89741b29e62059) review, this factors feature flags for Fabric core code into a separate file, `CoreFeatures`.
Keeping them together is arguably better for maintenance and makes code easier to reason about.
Changelog: [Internal]
Reviewed By: sammy-SC
Differential Revision: D40007784
fbshipit-source-id: 1885d5d6200575c6015f063d8b05813b18b47ffb