Deals with transforming a collection of markdown-it tokens obtained from parsing markdown text into an AST composed
of nested clojure structures.

See `parse` function.

A "Node" has the following keys:
- type: a Node's type as keyword (:heading, :paragraph, :text, :code etc.)
- info: (optional) fenced code info
- content: (optional) a collection of Nodes representing nested content
- text: (optional) content of text nodes, a collection of Nodes
- heading-level: (on `:heading` nodes)
- attrs: attributes as passed by markdownit tokens (e.g {:attrs {:style "some style info"}})

transform markdown data as returned by `nextjournal.markdown/parse` into other formats, currently:
* hiccup