Support `shadowrootdelegatesfocus` attribute

[michaelcpuckett]

shadowrootdelegatesfocus is an attribute that is part of Declarative Shadow DOM:

https://web.dev/declarative-shadow-dom/

Explainer: https://github.com/mfreed7/declarative-shadow-dom#additional-arguments-for-attachshadow

HTML Spec: https://html.spec.whatwg.org/multipage/scripting.html#the-template-element

This is implemented in Chrome but does not seem to be implemented in Safari.