Gets the tree of named destinations (string keys), or null when there is none.
public PdfNameTree? Dests { get; }
ISO 32000-2 §7.7.4, Table 32 (PDF 1.2), and §12.3.2.4: each value is a destination array, or a dictionary whose D entry
is one. Named destinations of PDF 1.1 (name keys) are in the catalog's own Dests dictionary instead.