Add Fragment Refs to Fabric with intersection observer support (#33056)

Adds Fragment Ref support to RN through the Fabric config, starting with
`observeUsing`/`unobserveUsing`. This is mostly a copy from the
implementation on DOM, and some of it can likely be shared in the future
but keeping it separate for now and we can refactor as we add more
features.

Added a basic test with Fabric, but testing specific methods requires so
much mocking that it doesn't seem valuable here.

I built Fabric and ran on the Catalyst app internally to test with
intersection observers end to end.