Add sketch for tree-sitter mode

[dnezam]

This PR contains an initial (and incomplete!) sketch for a tree-sitter based Emacs mode. There are still some things that do not get parsed, some things are probably expressed in an awkward way, and there is no indentation rules yet.

To use it, you will need to

1. Install tree-sitter
2. Call tree-sitter generate within the directory that contains the grammar
3. Call make
4. Copy the generated libtree-sitter-holscript.so file to your user-emacs-directory
5. Load the holscript-ts-mode.el file
6. Start the mode

If anything doesn't work, please let me know! I am more than happy to help wherever I can.

Some useful references:

• Mastering Emacs: How to Get Started with Tree-Sitter
• Mastering Emacs: Let’s Write a Tree-Sitter Major Mode
• tree-sitter documentation

[mn200]

Thanks!