Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/52474
1. Enables the feature flags for fragment refs on fbsource
2. Adds a test with Fantom for usage of `FragmentInstance#observeUsing` and `FragmentInstance#unobserveUsing`.
3. Exposes the `ReactFragmentInstance` type with the common methods that are used on native. We can override the DOM only methods in www libdefs.
Changelog: [Internal]
Reviewed By: rubennorte
Differential Revision: D74326262
fbshipit-source-id: e35ee45b23179ad3ba5527763567c9b04c127eff