Implement public instances for text nodes in Fabric (#26516)

## Summary

This adds the ability to create public instances for text nodes in
Fabric. The implementation for the public instances lives in React
Native (as it does for host components after #26437). The logic here
just handles their lazy instantiation when requested via
`getPublicInstanceFromInternalInstanceHandle`, which is called by Fabric
with information coming from the shadow tree.

It's important that the creation of public instances for text nodes is
done lazily to avoid regressing memory usage when unused. Instances for
text nodes are left intact if the public instance is never accessed.

This is necessary to implement access to text nodes in React Native as
explained in
https://github.com/react-native-community/discussions-and-proposals/pull/607

## How did you test this change?

Added unit tests (also fixed a test that was only testing the logic in a
mock :S).