[whatwg/dom] Upstream retargeting algorithm (#276)

See https://w3c.github.io/webcomponents/spec/shadow/#dfn-retargeting-algorithm. It seems Pointer Lock might use it soon per https://github.com/w3c/pointerlock/pull/4 and it's needed by `activeElement` too.

I wonder if we should figure out if event dispatch can somehow use it after all.

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