Problem: Currently default color scheme defines most of treesitter
highlight groups. This might be an issue for users defining their own
color scheme as it breaks the "fallback property" for some of groups.
Solution: Define less default treesitter groups; just enough for default
color scheme to be useful. That is:
- All first level groups (`@character`, `@string`, etc.).
- All `@xxx.builtin` groups as a most common subgroup.
- Some special cases (links/URLs, `@diff.xxx`, etc.).
1e0996b572
