Summary:
We double down on JSI in Fabric. So, practically, JSI is now a hard dependency for Fabric. I hope it's for good.
Now `jsi::Runtime` is coupled with scheduling via `EventExecuter`, so we have to make `jsi::Runtime` a part of `EventBeat` to proxy runtime reference to bindgings.
Reviewed By: sahrens
Differential Revision: D12837225
fbshipit-source-id: 98edc33d6a3358e6c2905f2f03ce0004a9ca0503