This adds an `--evaluate-only` option to `tree-sitter generate`
so that it only does the evaluation of `grammar.js` to
`src/grammar.json`, without continuing on with the generation of
`src/parser.c` and related files.
It's a follow-up to #4580.
* Move all rust crates (except lib) into crates dir, w/o nesting
* Remove stale path from .gitattributes
* Rename lib.rs files for easier navigation
* Rename mod.rs file for easier navigation
* Fix emscripten-version path
* Fix fixtures dir paths
* Use the default rustfmt settings
* Don't use nightly on CI
