fix: keep each table row's cells

A row with fewer cells than the header got empty cells that
pulldown-cmark makes up, and a row with more lost the extra ones,
which GFM ignores and pulldown-cmark drops. Prettier writes each row
with the cells it was written with, so mdtidy now does too: made-up
cells sit empty at the row's end and are skipped, and the text after
the last cell is split on unescaped pipes. A `|` inside a code span
splits a cell in GFM, so it can leave such extra cells.