fix: join wide lines like Prettier

When lines join in a paragraph, Prettier writes nothing for the line
break between two Chinese or Japanese characters, or next to their
punctuation, where mdtidy wrote a space. CSS renders such a break as
nothing between two wide characters (not Korean), so mdtidy now glues
them, and the semantics oracle compares such a break as nothing.

Prettier sometimes glues a break between Chinese or Japanese and Latin
text too, which CSS renders as a space; there mdtidy keeps a space
that doesn't break. In a reference link written as its label, the
break stays a line break, since gluing would change the label.