Summary:
Add `onStartReached` and `onStartReachedThreshold` to `VirtualizedList`. This allows implementing bidirectional paging.
## Changelog
[General] [Added] - Add onStartReached and onStartReachedThreshold to VirtualizedList
Pull Request resolved: https://github.com/facebook/react-native/pull/35321
Test Plan: Tested in the new RN tester example that the callback is triggered when close to the start of the list.
Reviewed By: yungsters
Differential Revision: D41653054
Pulled By: NickGerleman
fbshipit-source-id: 368b357fa0d83a43afb52a3f8df84a2fbbedc132