dillon1000/react

Make InternalInstanceHandle type opaque in ReactNativeTypes (#26461)

## Summary

This type was defined as `mixed` to avoid bringing the whole definition
from React to React Native, but its definition is visible to RN. This
type should be opaque to RN, so this makes it explicit.

## How did you test this change?

Applied the same changes in the React Native repository and could use
the type without issues.