Problem: Allowing undocumented "secret" (sub)captures makes it harder
to write comprehensive colorschemes and catch inconsistent captures.
Solution: Only allow captures listed in CONTRIBUTING.md. Add useful
(cross-language) subcaptures and drop language-specific or too niche
ones.
Follow-up: Adding further `*.builtin` captures and changing queries to
use them.
Language-specific subcaptures should instead be added in user config or
a custom language plugin.