ElementInternals should work with customized built-in elements

Spec: https://html.spec.whatwg.org/C/#dom-attachinternals

Now ElementInternals doesn't work with customized built-in elements because of the first step of attachInternals().

1. If element's is value is not null, then throw a "NotSupportedError" DOMException.

We can't support Custom State Pseudo Class for customized built-in elements because of this restriction.

Proposal: Remove the step 1.

We need to triage all of IDL attributes/operations of ElementInternals.

• Attributes/operations for Form-associated custom elements
  Add "Throw "NotSupportedError" DOMException if target element's "is" value is not null." to them

• Default focus behavior https://github.com/whatwg/html/pull/5120
  Ditto?

• Default accessibility semantics for custom elements https://github.com/whatwg/html/pull/4658
  I'm not sure if the current PR is reasonable for customized built-in elements. @domenic ?

• Custom State Pseudo Class
  It should work with customized build-in elements. We need to remove 'autonomous' from the specification.

Start at the HTML Standard's ElementInternals and attachInternals() algorithms, then read the linked proposals on default focus behavior and accessibility semantics. Review each listed IDL attribute and operation for customized built-in elements. Done means the specification's treatment of customized built-ins is resolved consistently across these areas.