Previously we used our own homespun formatting. But this
produces over-long lines that aren't ideal for diffs in tests.
Easier to use something off-the-shelf and standard.
Closes#7580.
Performance is slower by about a factor of 10, but this isn't
really a problem because native isn't suitable as a serialization
format. (For serialization you should use json, because the reader
is so much faster than native.)
Otherwise last block gets parsed as a Plain rather than
a Para.
This is a regression in pandoc 2.x. This patch restores
pandoc 1.19 behavior.
Closes#5271.
