Just pushed up a commit that I _think_ finishes this off - it could be pulled
into main.
- Using `HELIX_DEFAULT_RUNTIME` to point to the `/usr/pkg/share/helix/
runtime/` directory. Means we don't have to patch lib.rs
- Uses `CARGO_MANIFEST_DIR` during the grammar build to ensure they get
built - otherwise if you already have Helix installed and are using a
runtime directory that can override things during the build and it'll
think the grammars are already built.
- At some point in the future it might be nice to use Python to process
the languages.toml file, but for now we keep awk.