## Summary
Currently when cloning nodes in Fabric, we reset a node's children on
each clone, and then repeatedly call appendChild to restore the previous
list of children (even if it was quasi-identical to before). This causes
unnecessary invalidation of the layout state in Fabric's ShadowNode data
(which in turn may require additional yoga clones) and extra JSI calls.
This PR adds a feature flag to pass in the children as part of the clone
call, so Fabric always has a complete view of the node that's being
mutated.
This feature flag requires matching changes in the react-native repo:
https://github.com/facebook/react-native/pull/39817
## How did you test this change?
Unit test added demonstrates the new behaviour
```
yarn test -r www-modern ReactFabric-test
yarn test ReactFabric-test.internal
```
Tested a manual sync into React Native and verified core surfaces render
correctly.
151e75a128