[suggestion] make lockParentWindow work on MacOS

[stephane-archer]

  /// If [lockParentWindow] is set, the child window (file picker window) will
  /// stay in front of the Flutter window until it is closed (like a modal
  /// window). This parameter works only on Windows desktop.
  Future<String?> saveFile({
    String? dialogTitle,
    String? fileName,
    String? initialDirectory,
    FileType type = FileType.any,
    List<String>? allowedExtensions,
    Uint8List? bytes,
    bool lockParentWindow = false,
  }) async =>