Add quarto markdown processing option

[hughjonesd]

Quarto lets you use \QuartoMarkdownBase64{content} and <span data-qmd-base64="content" to include markdown in HTML and TeX. This will then be processed by the quarto engine. (You need to call base64enc::base64encode(charToRaw("content")) on the content.)

We could add this as an argument to set_markdown_contents(); and/or as a global option for markdown cells. This would then enable cool stuff like @citation, footnotes and references.

We're already using this to process captions, but this is more as a workaround for a bug in quarto 1.5 where labels get deleted; see #254 and the linked quarto reference. The base-64 encoded LaTeX is presumably interpreted as "latex within markdown".