windowing library

Bridges Flutter's experimental multi-window API to nativeapi.

Kept out of package:nativeapi/nativeapi.dart because it imports Flutter's internal windowing libraries, whose names still change between releases; it follows the stable channel (checked with Flutter 3.47.5). Apps that never import this library are unaffected by changes to those internals.

The stable channel does not offer flutter config --enable-windowing: an app sets isWindowingEnabled = true (from package:flutter/src/foundation/_features.dart) before its binding starts.