Add getRootNode to fabric fragment instance (#34544)

Stacked on #34533 for root fragment handling

This is the same approach as DOM, where we call getRootNode on the
parent.

Tests are in react-native using Fantom.

DiffTrain build for [e866b1d1e9b8a1dc7a43f1be0c510784566b9bb1](https://github.com/facebook/react/commit/e866b1d1e9b8a1dc7a43f1be0c510784566b9bb1)