Gets the program's built-in encoding: the glyph name of each of the 256 character codes, .notdef where it maps none;
null when the format has no built-in encoding (TrueType) or the program records none.
public virtual IReadOnlyList<string>? BuiltInEncoding { get; }
ISO 32000-2 §9.6.5.1 and §9.6.5.2: a Type 1 program's /Encoding (a CFF program's Encoding). A simple font with an
embedded program uses it when its Encoding entry gives no base encoding.