`shadowrootdelegatesfocus` attribute

Summary: A shadow host with `shadowrootdelegatesfocus` set to true will delegate its focus to its first focusable child.

Spec: https://dom.spec.whatwg.org/#shadowroot-delegates-focus

MDN: https://developer.mozilla.org/en-US/docs/Web/API/ShadowRoot/delegatesFocus

Other Resources:

• Introduce "delegatesFocus" flag to shadow root #126
• Design doc

=== This is a demo of shadowrootdelegatesfocus attribute. ===