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

My concerns with a getter/setter is a hard reference could leave an object in memory, prohibiting it from being garbage collection. With DOM nodes, once a child is detached from its parent, `.parentNode` becomes `null` (or `undefined`, not sure). The parent can then be garbage collected if it's no longer referenced. That's one of the reasons why I leaned to a function.

The other idea I had with my custom implementation is a EventTarget parent registry of sorts. Basically, instead of the child explicitly stating what it's parent is, a global registry can be used. eg: `parent = EventTargetParentRegistry.get(child)`. Here, a user agent can basically keep a dictionary of sorts since no child can have multiple parents. 

A pure JavaScript solution (polyfill) could look something like this:

````js
class EventTargetParentRegistry {
  /** @type {WeakMap<EventTarget, WeakRef<EventTarget>>} */
  static #registry = new WeakMap();

  static get(child) {
    return this.#registry.get(child)?.deref();
  }

  static set(parent, child) {
    this.#registry.set(child, new WeakRef(parent));
  }

  static delete(child) {
    this.#registry.delete(child);
  }
}
````


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