XML: preserve node methods with matching children

XML: preserve node methods with matching children

QuickJS exposes child elements as own properties, so a child named
like an XMLNode method hides the method on its prototype. Let methods
take precedence over bare child names, as on the njs engine. Keep the
child accessible through $tag$name and avoid deleting it when deleting
the method name.

Cover collisions for all XMLNode methods and prototype changes.