Markdown backend: complete implementation

[lukstafi]

Currently, multiline preformatted content as separate inline code markers. Will consider using code blocks...

I'll also test links, complex combinations and big output sizes.

[lukstafi]

First experiments with ppx_minidebug look very promising, no issues! Sill need to:

• see if code blocks would look nicer than separate lines of inline code
• verify table export works (as text and as html)
• verify a large-scale ppx_minidebug use case

[lukstafi]

Let me know if you'd like to have printbox-md try approximately exporting special cases of tables in a more Markdown-friendly format. Meaning:

• using Markdown tables in the rare cases where they fit (bold-style or framed header and single line rows)
• using lists for single-column tables (vlist)
• using... the nbsp converter for single-row tables (hlist).

So, I noticed I get a lot of fluff because I've used vlist x in one place, and I can easily use tree empty x instead -- but maybe we can do that inside printbox-md itself.

[lukstafi]

I checked that GitHub's Markdown rendering is capped at below 2MB. VSCode Markdown rendering works with bigger files, but there's an annoying waiting period (in that there's no reaction, a progress indicator would make it more bearable) that's even longer than with GitHub.

[lukstafi]

• using lists for single-column tables (vlist)

I'll implement this simple case, since tree empty x causes an extra fold.

[lukstafi]

The biggest bummer is that GitHub Markdown rendering, as far as I see, just entirely rejects using styles provided by <div style=...> or <span style=...> elements. This is not expected! (Other renderers, and CommonMark, render styles as normal HTML.) Let's do this: I keep a personal branch with the full scope so I feel less bad about it, and remove the Stylized functionality from the PR.