Re: [webcomponents] Replace attached/detached callbacks with insertedIntoDocument/removedFromDocument (#362)

F2F agreement: let's do this. Delta from the current spec: `attached` -> `insertedIntoDocument`; `detached` -> `removedFromDocument`; remove the "_and_ this document has a browsing context" clause.

Order of callbacks should be root of tree (document element) to leaves, to be consistent with other callbacks.

Shadow DOM does not change the answer. If you are inserted into a shadow tree whose shadow root is not in document, you do not get called. When the shadow root gets inserted, you are called (in order).

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