chore: export MaterialVideoProgressBar widget.

Author: yujunetee

lib/chewie.dart

@@ -4,5 +4,6 @@ export 'src/chewie_player.dart';
 export 'src/chewie_progress_colors.dart';
 export 'src/cupertino/cupertino_controls.dart';
 export 'src/material/material_controls.dart';
+export 'src/material/material_progress_bar.dart';
 export 'src/material/material_desktop_controls.dart';
 export 'src/models/index.dart';