I have added some notes on getting started to the README.

`npm install` should be working now, let me look at the tests, I think they've just gotten out of date. Note that there's a known issue in the core tests whereby tree-sitter seems to get stuck in a loop on zero-length token.

EDIT: Yes @John Murray it's the core grammar that has the zero-length token, not the expr one! (corrected).

From the udl directory, you should be able to do tree-sitter build --wasm (I use emscripten on Windows via scoop rather than the docker tooling) followed by tree-sitter playground to test. As I mentioned to Dmitry, I'll add some notes about getting started with contributing, it's mostly per the tree-sitter parser development docs, but I've found a few things that help!