Gets the terminal field the widget belongs to: the field whose Kids lists it, or the field merged with it; null
when the document has no interactive form or the widget belongs to no field. The same instance the form's field tree holds, whose
Widgets lists this widget.
public PdfTerminalField? Field { get; }
ISO 32000-2 §12.7.2 and §12.5.6.19. A widget the tree from Fields does not reach is resolved through its Parent
chain with a WidgetNotInFieldTree diagnostic.