This adds rudimentary language support for the Nix expression language,
through tree-sitter-nix.

I spent a little bit of time trying to add support for one of the Nix
language servers too, but wasn't able to get any of them running
reliably without crashing, and so I've opted to stick to just the
tree-sitter grammar for now.


![image](https://github.com/zed-industries/zed/assets/285821/c770f3d8-6fa0-4083-9bf3-239cc78ca307)