Turns out that 'tree-sitter' is not wrapped to have node and a compiler
available, because nixpkgs assumes that grammars already include
generated files.
So wrap it manually and use that in our pre-commit/checks.
Introduce an overlay, and use the existing 'tree-sitter' package
machinery to extend it with this grammar.
Expose the extended 'tree-sitter' as part of the flake packages.