Gets the developer extensions the document declares (Extensions), in the order written; empty when none.
public IReadOnlyList<PdfDeveloperExtension> Extensions { get; }
ISO 32000-2 §7.12, Tables 48 and 49. A prefix whose value is an array (PDF 2.0) contributes one extension per dictionary in it.
Deviations are reported when this property is read: an entry that is not a dictionary (ExtensionsInvalid), a prefix value
that is neither a developer extensions dictionary nor an array of them or that lacks a required entry
(DeveloperExtensionInvalid), an extension whose BaseVersion is later than Version
(DeveloperExtensionBaseVersionLater, §7.12.4; a save raises the version to it), and indirect references where everything
"shall be direct objects" (ExtensionsNotDirect).