fix: spread loose items

Prettier treats a list item as loose when a blank line separates two
of its direct children, or the item from the next one, and then puts a
blank line between all of its blocks. The one exception is a nested
list, which keeps the source's spacing. mdtidy followed the source
for each pair of blocks instead.

The item now reads ahead to its end to decide. Blank lines inside a
nested list or a code block don't count, as in Prettier.