Switch to more elegant approach

[maelle]

If possible one day... e.g. if commonmark full loop possible, and something implemented for chunk options (currently in Commonmark XML the whole chunk header is "info" of a code block), then use xml2.

Cf the end of https://ropensci.org/technotes/2018/09/05/commonmark/

[maelle]

That said, if I used https://github.com/ropenscilabs/tinkr, we'd have to deal with losses whereas the current approach only changes the lines with chunk info.