add `useFocusScopeNode` hook

[iamsahilsonawane]

trafficstars

Added focus scope node for FocusScope widget.

class Example extends HookWidget {
  @override
  Widget build(BuildContext context) {
   final node = useFocusScopeNode();
   return FocusScope(autofocus: true, node: node, child: ...);        
  }
}

Fixes #316