[WICG/webcomponents] RfC: Expose owner root on shadow root instance (#921)

I've done a little digging, though admittedly not a super deep dive so I aplolgize if this has been brought up elsewhere. Would it make sense to add a `ownerRoot` property to a `ShadowRoot` instance? This language is aligned with `ownerDocument` but it would serve to provide access to the `DocumentOrShadowRoot` to which any given shadow root instance is attached. This could be very helpful for accessibility controls or for controls that need to somehow query their surroundings. A simple use case would be password fields that need to ensure data matches between two components or if an element wants to query from a datalist:

```html
<datalist id="list">
  <!-- options -->
</datalist>

<label for="foo">Naive combobox example</label>
<x-combobox list="list" id="foo"></x-combobox>

<script>
class Combobox extends HTMLElement {
  #internals;
  constructor() {
    super();
    this.#internals = this.attachInternals();
    this.attachShadow({ mode: 'open' });
    this.#init();
  }

  #init() {
    const options = this.shadowRoot.ownerRoot.querySelector(`#${this.getAttribute('list')}`);
    // parse options
  }
}
customElements.define('x-combobox, Combobox);
</script>
```

I have a couple more complex use cases for this that right now I'm walking the parent tree until I find a shadow host which is easy enough, but I'm sure there are a number of other use cases out there and this could be a nice quality-of-life improvement.

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