Update tree-sitter highlight groups

[savq]

• Add @comment.documentation.
• Add explicit link from Boolean to Number.
• Highlight @string.special directly, instead of through SpecialChar.
• Don't italicize @string.regex or @string.documentation.
• Don't highlight @constant. Most patterns apply to all caps identifiers, i.e. not proper literals.
• Add @text.quote.
• Update @text.reference (underline instead of highlight).