[node_io] Implement `FileSystemEntity.watch` using `fs.watch` or `fs.watchDir`

[nikeokoronkwo]

I was trying to watch for events in a folder, and noticed that the watch method is not implemented.

@override
  Stream<io.FileSystemEvent> watch(
      {int events = io.FileSystemEvent.all, bool recursive = false}) {
    // TODO: implement watch
    throw UnimplementedError();
  }

I was thinking maybe this could be implemented using Node's fs.watch and fs.watchDir.