[RFC] Add a way to opt-in ext/dom spec compliance (#13031)

RFC: https://wiki.php.net/rfc/opt_in_dom_spec_compliance

Fix DOMElement::append() and DOMElement::prepend() hierarchy checks

We could end up in an invalid hierarchy, resulting in infinite loops and
eventual crashes if we don't check for the DO

Fix DOMElement::append() and DOMElement::prepend() hierarchy checks

We could end up in an invalid hierarchy, resulting in infinite loops and
eventual crashes if we don't check for the DOM hierarchy validity.

Closes GH-11344.

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