XML: return complete attribute values

XML: return complete attribute values

Previously, $attr$ returned undefined for attributes containing entity
references, while $attrs returned only the first text fragment. libxml2
canonicalization expands the complete attribute value, creating an
interpretation difference for signed XML consumers.

Use xmlNodeGetContent() for both getters in the njs and QuickJS backends.
This preserves the existing explicit-attribute lookup and makes the
JavaScript value agree with canonicalization.