cubicweb #15438011 [markdown] add support for non-standard (gitlab inspiered) code formating [open]

On gitlab issues when pasting some traceback or some code one does not have to indent the code, just surround it by

numerous lines of code

and it works out of the box. This avoids the need to copy/paste in an editor (emacs in my case) to indent by a few spaces the code for ReST or markdown.

done in<not specified>
closed by<not specified>