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.
...even when we must lose width information.
All in all this seems to be people's preferred behavior, even though it
is slightly lossier.
Closes#2608.
Closes#4497.
