Introducing a new tree view implementation

Hi! I love Cursive's design, thank you for this library.

The README currently links to [this](https://github.com/BonsaiDen/cursive_tree_view) tree view implementation, but it seems to have been abandoned a while ago. The maintainers do not respond. Also, note that it has some bugs and design limitations and is also made for a rather old version of Cursive.

I'm introducing [cursive-tree](https://codeberg.org/tliron/cursive-tree) as an up-to-date alternative. If there's interest, perhaps it can be added to the main README.

At the very least I do think the README should note that cursive_tree_view has been abandoned.