Auto merge of #20740 - fabricedesre:mutation-observer-disconnect, r=jdm

Implement MutationObserver.disconnect()

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This implements https://dom.spec.whatwg.org/#dom-mutationobserver-disconnect
I added a `node_list` to the `MutationObserver` struct to keep track of the nodes involved in an observer because otherwise I could not find a way to unregister the observers on the node themselves without traversing the whole document tree. Let me know if there's something I missed.

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