Fix #47531: No way of removing redundant xmlns: declarations

Now it's possible via removeAttribute("xmlns:prefix").
It was not possible to reuse a libxml2 function to reconcile because i

Fix #47531: No way of removing redundant xmlns: declarations

Now it's possible via removeAttribute("xmlns:prefix").
It was not possible to reuse a libxml2 function to reconcile because it
does not align with DOM behaviour.

Closes GH-12542.

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Implement DOMElement::toggleAttribute()

ref: https://dom.spec.whatwg.org/#dom-element-toggleattribute

Closes GH-11696.