Expose new method in Fabric renderer to access root instance from root tag (#49011)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/49011

Changelog: [internal]

This exposes the new `getPublicInstanceFromRoot` method from the React renderer in our RN façades, preparing for the new change to implement the document interface in RN.

Reviewed By: javache

Differential Revision: D68767143

fbshipit-source-id: 9a3403f9bc1612b402305695d084497a46ee4480