Add Dom\Element::$outerHTML getter

Reference: https://html.spec.whatwg.org/multipage/dynamic-markup-insertion.html#the-outerhtml-property

Fix GH-15192: Segmentation fault in dom extension (html5_serializer)

When cloning a document, doc will not be equal to the actual new
document clone->doc. clone->doc will always point to

Fix GH-15192: Segmentation fault in dom extension (html5_serializer)

When cloning a document, doc will not be equal to the actual new
document clone->doc. clone->doc will always point to the correct
document so use that instead when comparing document nodes.

Closes GH-15198.

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