[dom] Disconnect single target instead of all (#126)

Current specification of MutationObserver allows users to connect an observer to multiple target root nodes, but `disconnect` always acts on all of them.

It makes it hard to write systems that modify targets in reaction to mutations because they need to disconnect the target on which the mutation happened, modify, and reconnect.

A simple solution would be to add a single parameter to `disconnect` with targetNode.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/dom/issues/126