Make nix flake respect unused grammars

[loewenheim]

17dd102 marked some grammars as "do not use", but the nix flake doesn't respect this setting. This is my attempt to rectify this situation.

[the-mikedavis]

Other than except, only is also possible and I think we should have grammars.nix respect it even if we don't currently use it in languages.toml: https://docs.helix-editor.com/languages.html#choosing-grammars

Here's where we do it in the source:

helix/helix-loader/src/grammar.rs

Lines 208 to 220 in 17dd102

	let grammars = match config.grammar_selection {
	Some(GrammarSelection::Only { only: selections }) => config
	.grammar
	.into_iter()
	.filter(|grammar| selections.contains(&grammar.grammar_id))
	.collect(),
	Some(GrammarSelection::Except { except: rejections }) => config
	.grammar
	.into_iter()
	.filter(|grammar| !rejections.contains(&grammar.grammar_id))
	.collect(),
	None => config.grammar,
	};

I believe if both only and except are defined then we prefer only (ignoring except).

[loewenheim]

Thanks for the heads-up, will refactor.

[the-mikedavis]

Thanks!