[element] Scoped element registrations

[justinfagnani]

We need to support the upcoming scoped custom element registry spec.

A few things to do:

• Make <h-define-element> able to be non-self-registering
• Allow <h-define-element> to register elements within a scope. This probably looks like an element that can reference a constructor in various ways:
  • An id to another <h-define-element>
  • A module specifier/export combo
  • A Heximal expression
  • An HTML import and id reference
• Allow main document registrations of non-self-registering elements. This can probably use the same element as in <h-define-element>
• Possibly scope main document registrations within <h-scope>. The new scoped custom element registry spec allows this.

We will end up having two element definition-related elements: one declares the constructor, the other registers the element. Along with #26 maybe we have <h-define> and <h-register>. The corresponding JSs APIs are a class declaration and CustomElementRegistry.define() though, so maybe we want <h-declare> (make the class) and <h-define> (register the element). We'll need to ask potential users what's the most clear.