compute_list_marker(
tss,depth,list,item_count,task_info) ->marker_text,marker_width
Compute list marker text, width, and continuation indent
Computes the rendered list marker for a list item. list must have
ordered (bool) and start (number) fields. task_info, when non-nil,
must have a checked field. Returns the styled marker text (including
base indent) and its display width.