Add the Flutter Tree Extension from Marcelo Velasquez to the open-vsx store

Please add the Flutter Tree extension to the open-vsx store. The publisher was asked to do this but until right now, he didn't do it. He was asked to do this on Jun 30, 2020.
[GitHub Repository](https://github.com/itmarck/flutter-tree)
[Request to publish to open-vsx](https://github.com/itmarck/flutter-tree/issues/5)