Re: [w3c/webcomponents] Scoped Custom Element Registries (#716)

What happens in `my-foo` from the above example is removed then appended into another tree unrelated to the scope? Does it just continue to work? Or is an error thrown?

> fully composable registry graph where the resolution of a name can be delegated to any registry where the logic can be customized in user-land.

This seems like it could be useful for some sort of framework, but would definitely be nice to have a default (that just looks in the parent scope) so that the end user can just do something perhaps as easy as

```js
const myRegistry = new CustomElementRegistry()
myRegistry.define('my-foo', MyFoo);
this.root = this.attachShadow({mode: 'open', registry: myRegistry})
```

which causes lookup to look in the parent scope (parent shadow root) when the element name is not found in the current registry.

Just tossing in a syntax idea:


```js
this.root = this.attachShadow({mode: 'open', registry: true})
this.root.define('my-foo', MyFoo);
```

or maybe just simply:

```js
this.root = this.attachShadow({mode: 'open'})
this.root.define('my-foo', MyFoo); // creates a registry internally on first use
```

And for advanced use (f.e. defining lookup):


```js
this.root = this.attachShadow({mode: 'open'})

console.log(this.root.registry) // null

this.root.define('my-foo', MyFoo); // creates a registry on first use

console.log(this.root.registry) // CustomElementsRegistry

// ... and for advanced users:
this.root.registry.defineLookup(function() { ... })
```

This way it's easier, yet still configurable for advanced cases.

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