Gets the standard structure type the element's type resolves to through the role maps, or null when it resolves to none.
public PdfStructureType? StandardType { get; }
ISO 32000-2 §14.7.3, §14.7.4.2, §14.8.6.2; ISO/TS 32005 §5.6. In a tagged PDF (Marked) every element shall
resolve to a standard type (§14.8.4.1); one that does not is a StructureTypeUnresolved diagnostic. See
RoleMapping for the chain followed.