Provide tree-sitter grammar for mcfunction

[SPGoding]

Many text editors rely on Tree-sitter grammars (and queries) instead of TextMate-like regex for syntax highlighting:

• Atom: https://flight-manual.atom-editor.cc/hacking-atom/sections/creating-a-grammar/
• Emacs: https://git.savannah.gnu.org/cgit/emacs.git/tree/admin/notes/tree-sitter/starter-guide
• Helix: https://docs.helix-editor.com/master/guides/adding_languages.html#grammar-configuration
• Neovim: https://neovim.io/doc/user/treesitter.html
• Zed: https://zed.dev/docs/adding-new-languages#syntax-highlighting

Tree-sitter grammars are written in JS, and the Spyglass language server is intended to be compatible with all LSP-compliant text editors, so it feels within scope and maintainable to include a Tree-sitter grammar for mcfunction in this repo to help further improve the developer experience of data packs across all editors.

I wrote a grammar on the tree-sitter-mcfunction branch some time ago which can probably be a start point after v4 is closer to ready.