Re: [whatwg/dom] Make it possible to observe connected-ness of a node (#533)

LeaVerou left a comment (whatwg/dom#533)

Actually, I’m not sure if observing connectedness is what we need. Consider this:

```js
let parser = new DOMParser();
let doc = (new DOMParser).parseFromString("<p>Hi</p>", "text/html");
let p = doc.querySelector("p");
console.log(p.isConnected); // true
```

It seems that instead what most use cases need is to observe (a) `ownerDocument` changes and (b) `parentNode` changes. 

Then we can get all the existing custom element lifecycle hooks:
- `connectedCallback()` → `ownerDocument` goes from `null` to a non-null value
- `disconnectedCallback()` → `ownerDocument` changes from a non-null value to `null`
- `adoptedCallback()` → `ownerDocument` changes from a non-null value to a different non-null value. 
- `connectedMoveCallback()` → `parentNode` changes from a non-null value to another non-null value, while `ownerDocument` remains the same

Plus introducing such properties to `MutationObserverInit` is also much more in line with its existing API where options describe the lower-level tree variables. Connectedness is a derived variable.

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