[w3c/webcomponents] CustomElementRegistry per shadow DOM (#792)

I think custom elements should be able to be defined per shadow DOM like css.

```js
window.customElements.define('my-custom-element', MyCustomElement0);
shadowroot1.customElements.define('my-custom-element', MyCustomElement1);
shadowroot2.customElements.define('my-custom-element', MyCustomElement2);
```

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