dillon1000/react

Clean interface for public instances between React and React Native (#26416)

## Summary

We are going to move the definition of public instances from React to
React Native to have them together with the native methods in Fabric
that they invoke. This will allow us to have a better type safety
between them and iterate faster on the implementation of this proposal:
https://github.com/react-native-community/discussions-and-proposals/pull/607

The interface between React and React Native would look like this after
this change and a following PR (#26418):

React → React Native:
```javascript
ReactNativePrivateInterface.createPublicInstance // to provide via refs
ReactNativePrivateInterface.getNodeFromPublicInstance // for DevTools, commands, etc.
ReactNativePrivateInterface.getNativeTagFromPublicInstance // to implement `findNodeHandle`
```

React Native → React (ReactFabric):
```javascript
ReactFabric.getNodeFromInternalInstanceHandle // to get most recent node to call into native
ReactFabric.getPublicInstanceFromInternalInstanceHandle // to get public instances from results from native
```

## How did you test this change?

Flow
Existing unit tests