Parse the XML declaration's pseudo-attributes instead of skipping the
<?xml?> processing instruction:
- version, encoding, and standalone are surfaced on a new XMLDeclaration,
returned from parseWithDocumentType and via PureXML.xmlDeclaration(_:).
- The pseudo-attributes are validated: fixed order (version, encoding,
standalone), known names only, and standalone must be yes or no. A
malformed declaration is rejected.
- The declared version is now readable, a prerequisite for XML 1.1.
parseWithDocumentType returns a ParsedDocument struct (node,
documentType, declaration) rather than a 2-tuple; the member names are
unchanged so existing call sites are unaffected.
Closes#95
