Gets a value indicating whether the font is bold: the ForceBold flag, a FontWeight of 600 or more, a bold word in the
name (Bold, Semibold, Demi, Black, Heavy), or, when neither weight nor name says, a StemV of 120 or more.
public bool IsBold { get; }
ISO 32000-2 §9.8.1 and §9.8.2. The StemV threshold is this library's (Helvetica 88, Helvetica-Bold 140, Times-Bold 139).