Gets the field flags (Ff, inheritable), every bit as stored; 0 when absent.
public PdfFieldFlags Flags { get; }
ISO 32000-2 §12.7.4.1, Tables 226 and 227: an unsigned 32-bit integer. The value of the nearest dictionary that has the entry
is used whole, never combined with an ancestor's. A real number is truncated and a negative integer read as its 32-bit pattern,
with a FieldFlagsInvalid diagnostic; so is a bit no field type defines, which is kept. The type-specific bits are the
properties of each field class.