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

Deno actually uses a somewhat custom method of "get the parent" that allows custom overloading. The check occurs by checking if the `EventTarget` "is a node" by checking if has a `nodeType` property. Then it uses `.parentNode`. That means an Javascript Object that has `nodeType` can potentially return a parent (via `.parentNode`).

https://github.com/denoland/deno/blob/704e1e53307130012da974f42d2cad94e07b5664/op_crates/web/02_event.js#L438

This is piggybacking off `Node`, but to write a proper shim for it (I've tried it), you have to implement `Node`, `ChildNode`, `ParentNode`, `Document`. and some others that my escape me at the moment. That would be a lot of work for user agents to include and force them to require support something far more complicated just `EventTarget`. The Deno is using something custom as is for lack of a standard, and I can totally understand doing it like that instead of writing all of `Document`.

I can only realistically conclude that the EventTarget may be constructed with a `eventTargetInit` object that may have a `get the parent` value. So you can maybe do something like `new EventTarget({ getParent: myMasterParentFn })`, for example. It keeps it strictly within `EventTarget` without telling user agents to write more code, or having developers need to add on something heavy like JSDom.

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