[webcomponents] Disallow extension of subclasses of HTMLElement and SVGElement/MathMLElement (#363)

As summarized in https://wiki.whatwg.org/wiki/Custom_Elements#Subclassing_existing_elements, subclassing existing built-in subclasses elements of `HTMLElement` pose many issues.  We should remove this feature (`extends` on the custom element definition and `is` content attribute) at least from v1.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/363