Treat runtime files from installation as immutable

[the-mikedavis]

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We should treat the grammar sources, compiled parsers, and queries produced while building Helix as immutable defaults which can be overridden with the existence of files in ~/.config/helix/runtime. Effectively, $HELIX_RUNTIME should act like $PATH in unix.

Except when installing from source, these files almost always end up in some /usr directory that needs sudo to edit (which you probably shouldn't do). With Nix, these end up in the store which is immutable so you need to build from source to modify or queries locally. Instead you should just be able to override files in your ~/.config/helix/runtime directory.

See https://github.com/helix-editor/helix/issues/3310#issuecomment-1203875141 See https://github.com/helix-editor/helix/issues/2091#issuecomment-1097472407