Re: [w3c/webcomponents] Expose shadowRoot on element internals (#871)

I thought I'd update the code examples from https://github.com/w3c/webcomponents/issues/871#issuecomment-604741570.

```js
class CustomHTMLElement extends HTMLElement {
 #internals = this.attachInternals()
 #shadow = this.#internals.shadowRoot ?? this.attachShadow({mode: 'closed'})
}
```

vs

```js
class CustomHTMLElement extends HTMLElement {
 #existingShadow = this.attachShadow({returnExistingDeclarativeShadowRoot: true})
 #shadow = this.#existingShadow ?? this.attachShadow({mode: 'closed'})
}
```

And if @rniwa you meant that a single call to `attachShadow` would create a new root with the given mode only if an existing one doesn't exist, then your original example simplifies to

```js
class CustomHTMLElement extends HTMLElement {
 #shadow = this.attachShadow({mode: 'closed', returnExistingDeclarativeShadowRoot: true})
}
```

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