fix: keep html block indents

Prettier keeps the indent of an HTML block's first line, up to three
spaces. pulldown-cmark reports that indent as a text event with an
empty range, which mdtidy dropped. Inside an HTML block it is now
written.

The indent stays below the content indent of a list that just ended
at the same depth. Otherwise, where the Preserve preset narrows a
marker like `1.  a` to `1. a`, the HTML block would join the list.