Problem: Many parsers require node/npm to evaluate the `grammar.js`
before being able to generate a parser from it.
Solution: Generate from `grammar.json` instead, which is fully resolved.
Drops `node` and `npm` as (optional) requirements for nvim-treesitter.
Note that this requires parsers to commit the generated json iff the
grammar requires evaluation (which is currently the case for all tracked
languages).