Switch to commonmark_x as default

[tarleb]

Not sure if this is too early, but with pandoc 3 in the works, would it make sense to seize the opportunity and switch the default format to commonmark_x?

[jgm]

It is too early, I think, because unfortunately commonmark_x doesn't have a citations extension. (It also lacks grid tables and lots of other things.) So I think this would be a major breaking change for a lot of people, and not worth it.

I've had a devil of a time designing a commonmark-hs extension that deals with citations.