Gets how the items are numbered, as a ListNumbering value: None, Unordered, Description,
Disc, Circle, Square, Ordered, Decimal, UpperRoman, LowerRoman,
UpperAlpha or LowerAlpha. For a tagged list it is the element's attribute in effect (None by default, or
whatever name the file holds); for a detected list it is read from the items' markers (Unordered for a bullet that is
not a disc, circle or square).
public string Numbering { get; }
ISO 32000-2 §14.8.5.5, Table 385 (ListNumbering; PDF 2.0 adds Unordered, Description and Ordered).