Ordered lists nest with letters and roman numerals, and list items get a hanging indent

Markdown rendering now computes a marker per list item and tracks a listIndent as lists nest. A second-level ordered list numbers its items as a/b/c, and a third-level list uses roman numerals, falling back to plain digits once the last item's number exceeds 3999.

Continuation lines within a list item are now indented to line up under that item's marker, except when the item opens with a code block, blockquote, horizontal rule, table, or a nested list, in which case the old formatting is kept.