Wire up @codemirror/lang-markdown with a custom HighlightStyle to
highlight headings, bold, italic, strikethrough, links, lists,
blockquotes, and inline code in the raw CodeMirror editor. The custom
frontmatter plugin is kept for YAML highlighting; its heading
decoration is removed in favour of the language-based parser.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
