Re: [WICG/webcomponents] Scoped Custom Element Registry: Moving elements with shadow roots between documents (#907)

An alternative to throwing could be to have a single element name reserved for webcomponents that are not in the global registry e.g. something like `<custom>` or something. When an element is removed from a registry scope it's name would just become `custom`, but otherwise it would behave the same.

e.g.:

```js
const registry = new CustomElementRegistry();
registry.define("foo-bar", FooBarElement);
const shadowRoot = someElement.attachShadow({ registry, mode });

shadowRoot.innerHTML = `<foo-bar baz="qux"></foo-bar>`;

// Inside the shadow root it has it's expected name
const el = shadowRoot.children[0]
el.localName === "foo-bar"; // true

el.remove();
document.body.append(el);

// Inside other roots where FooBarElement is not registered it becomes <custom>
el.localName === "custom"; // true

// <custom> elements created normally would not do anything special and could not be upgraded
const custom = document.createElement("custom");
custom.constructor === HTMLElement; // true
```

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