Extend the core parser and the CLI renderer to cover the everyday Markdown
body beyond headings and paragraphs: lists, blockquotes, fenced code,
thematic breaks, and inline emphasis, strong, code spans, and links.
Add remark-gfm to the pipeline so tables and strikethrough parse as their
own nodes. Any block or inline node core does not model falls through to
RawBlock / RawInline, whose verbatim value is position-sliced from the
original input so unmodelled constructs round-trip byte-for-byte.
