We do not highlight `(ERROR)` nodes since the highlighting is quite
noisy while typing. Also see todo comments in `syntax.rs` - we could
introduce configuration in the future to prepend `(ERROR)` to a
language's highlights query.
- Rename markup.underline.link to markup.link.url
- Add markup.link.label
- Add markup.quote
(The constructor theme scope was missing from the
docs, so unrelated to this commit).