Add feature flag to use microtasks in the React Native Fabric renderer (#27364)

## Summary

This is part of an effort to align the event loop in React Native with
its behavior on the Web. In this case, we're going to test enabling
microtasks in React Native (Fabric) and we need React to schedule work
using microtasks if available there. This just adds a feature flag to
configure that behavior at runtime.

## How did you test this change?

* Reviewed the generated code, which looks ok.
* Did a manual sync of this PR to Meta's internal infra and tested it
with my changes to enable microtasks in RN/Hermes.