[WICG/webcomponents] New Lifecycle Callback: `willConnectCallback` for initializing attributes (#919)

Well, we can't set attributes in constructor, and if we set them in `connectedCallback` it will cause some problems with `MutationObserver` (e.g. when a custom element is added to DOM, first mutationCallback is triggered then attributes are initialized in `connectedCallback`).
So what about a lifecycle callback before element is added to DOM that is ergonomically the best way to initialize attributes?

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