Instead rely on the markdown writer with appropriate extensions.
Export writeCommonMark variant from Markdown writer.
This changes a few small things in rendering markdown,
e.g. w/r/t requiring backslashes before spaces inside
super/subscripts.
Previously we got a crash, because we were trying to print
a native cmark STRIKETHROUGH node, and the commonmark writer
in cmark-github doesn't support this. Work around this by
using a raw node to add the strikethrough delimiters.
Closes#4038.