Tab now indents a list item to its parent's content column, so a numbered
child becomes a CommonMark nested list. A new child list starts at 1 and an
existing child list continues its numbering. Enter on an empty nested item
lifts it one level before leaving the list.

Adapted from #17138 by michal-billtech.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>