Implement `getHTML()` serialization protocol

See HTML Standard [Serializing HTML fragments](https://html.spec.whatwg.org/multipage/parsing.html#serialising-html-fragments)

Discussion https://github.com/whatwg/html/issues/8867

Shadow Root interface extension https://github.com/whatwg/dom/commit/2d890bd2a4685df83c50f54eae77c13bf3bb2bd2

Mozilla implementation https://bugzilla.mozilla.org/show_bug.cgi?id=1890428