The vision with 'use-grammars' is to allow the long-requested feature
of being able to declare your own set of grammars that you would like.
A simple schema with only/except grammar names controls the list
of grammars that is fetched and built. It does not (yet) control which
grammars may be loaded at runtime if they already exist.
If a user has a `languages.toml`, only the grammars in that `languages.toml` are evaluated when running `hx --fetch-grammars` and `hx --build-grammars`. Otherwise the [default `languages.toml`](https://github.com/helix-editor/helix/blob/master/languages.toml) is used.
You may use a top-level `use-grammars` key to control which grammars are fetched and built.
```toml
# Note: this key must come **before** the [[language]] and [[grammar]] sections