[webcomponents] Permit retrieval of registered classes from window.customElements (#445)

Following up on issue #431, I'd like to propose that window.customElements allow a dev to retrieve information about the custom elements which have been registered. It sounds like @domenic was supportive of this idea, so as a strawman, I propose adding the following:

* `window.customElements.get(name)`. Returns the class which was previously registered with `define(name)`, or `undefined` if no element has yet been registered with that name.
* `window.customElements[@@iterator]()`. This returns a new `Iterator` object that enumerates the `[name, class]` pairs registered. I don't particularly care in what order the entries are returned. For consistency with `Map.prototype[@@iterator]()`, it would be nice to return these in the order in which they were defined.

This, at least would meet the needs I've encountered, chiefly being able to: 1) easily verify that a particular custom element has in fact been registered, 2) conditionally define an element if it hasn't already been registered. E.g.:

    if (!window.customElements.get('my-element')) {
      window.customElements.define('my-element', MyElement);
    }


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