fix: drop nested list blanks

Prettier drops the blank line before a nested list unless a paragraph
or code comes before it. mdtidy kept it whenever it was the only blank
in the item, but a loose item also gets a blank line between any two
children where the second is not a list, so the list stays loose and
the line can go. It still stays where dropping it would make the list
tight, which changes its paragraphs; that case is now a meaning
divergence.