Resurrect the tree-sitter grammar

[kritzcreek]

I'd like to have a tree-sitter grammar for Nemo for easy highlighting in this repository, so I don't have to juggle two repos.

To have that happen we'd need to figure out how to automatically generate tree-sitter test cases from our existing compiler tests.
I also don't want to commit any of the generated C files.

Grammar lives in https://github.com/kritzcreek/tree-sitter-nemo at the moment.