Re: [WICG/webcomponents] [scoped-registries] Interaction with declarative shadow DOM (#914)

The "winning proposal" has a (minor?) downside that we can't reuse registry for different shadow roots, even for the same component. So we need to create duplicate registries. For example:

```html
<some-element>
   <template shadowrootmode="open" customregistry>
      <some-scoped-element></some-scoped-element>
      <some-other-scoped-element></some-other-scoped-element>
   </template>
</some-element>
<script type="module">
  class SomeElement extends HTMLElement {
    constructor() {
      super();
      if (this.shadowRoot) {
        // This is duplicated on every instance of SomeElement
        this.shadowRoot.registry.define('some-scoped-element', SomeScopedElement);
        this.shadowRoot.registry.define('some-other-scoped-element', SomeOtherScopedElement);
      } else { ... }
    }
  }
 customElements.define('some-element', SomeElement);
</script>
```

Not sure how big of an issue it is, since declarative shadow DOM already needs to duplicate the markup and other things. But we can still avoid this by allowing setting `ShadowRoot.registry` if the `ShadowRoot` currently has a non-null but empty registry:

```html
<some-element>
   <template shadowrootmode="open" customregistry>
      <some-scoped-element></some-scoped-element>
      <some-other-scoped-element></some-other-scoped-element>
   </template>
</some-element>
<script type="module">
  import { registry } from './some-element-registry.mjs';
  class SomeElement extends HTMLElement {
    constructor() {
      super();
      if (this.shadowRoot) {
        // This makes all SomeElement instances share the same registry
        this.shadowRoot.registry = registry;
      } else { ... }
    }
  }
 customElements.define('some-element', SomeElement);
</script>
```

I think it has been debated many times at various places whether we should allow setting `ShadowRoot.registry`, and the general consensus is no, as changing definitions with already upgraded elements can be quite problematic. But if the scoped registry is empty, then I don't see any issue -- it's the same as duplicating the definitions from the other registry. So I don't really see any downside from this proposal, and it's a free performance gain.

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