[whatwg/dom] Allow custom "get the parent" algorithms for EventTargets. (#583)

> In the future we could allow custom get the parent algorithms. Let us know if this would be useful for your programs. For now, all author-created EventTargets do not participate in a tree structure.

I'd like to see that future. That is, the ability to create a tree structure for author-created EventTargets like the one we have with Nodes, with bubbling and capturing, and so forth. Two points in support of this:
1. It would just make the platform more consistent when all EventTargets behave similar or have ways to do so. Right now we have a "weird" limitation to keep in mind when we work with custom EventTargets.
2. Listening just to a parent instead of each of its children simplifies our code when we work with Nodes, and same can apply to author-created EventTargets.

An example use case:
Let's take the classic Todo app example. Imagine that each todo item is a custom EventTarget emitting "change" events, and the whole list is an array. Now, if we need to respond to any change in the list, say, recalculate the number of items, we have to add event listeners to each item. However, if we could define our array as the parent for each item where events could bubble to (or be captured on) the parent, we could get away with just one listener on the array. Since the items and the array already have strong coupling, adding an extra "parent" pointer wouldn't make difference in terms of the design.

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