[w3c/webcomponents] Add dispatcherEvent for callbacks (#489)

Hello! I was writing a notification example, something about "submit" a form within a custom element. And I noticed that there's one case when shadowRoot is null when creating. This happen when the custom element is unresolved.

What about if add an event that will be fired when the createdCallback was called?
Check [this example](https://github.com/rianby64/proposal1/blob/master/tag-a.js#L18)

I tried to solve the problem when shadowRoot is null by dispatching my own event.
Check [this example](https://github.com/rianby64/proposal1/blob/master/tag-b.js#L27)

Hope this idea will be useful in some way.
Thanks a lot.


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