Gets the actions performed after this one (Next, PDF 1.2), in order: none, the single action dictionary, or the
action dictionaries of the array. Each may have a Next of its own; see EnumerateActionTree(Int32).
public IReadOnlyList<PdfAction> Next { get; }
ISO 32000-2 §12.6.2, Table 196 and Note 1. Read from the dictionary on every call. An element that is not an action dictionary
is skipped with an ActionNextInvalid diagnostic, as is a Next that is neither a dictionary nor an array.