dillon1000/react

Fix errors in React Native children management (#9449)

* Fix errors in React Native children management

- Support using appendChild to move an existing child (Fiber does this, but we were assuming all children here were new; Yoga throws if you insert a child that already has a parent)
- Calculate beforeChildIndex after removing old child (previously, off by one when the new position is later than the old position)

* Add better children management tests for RN