Re: [webcomponents] Disallow attachShadow(...) for some elements. (#110)

BTW, I have also `button[is=cs-button]` element which also exposes some styling via CSS variables/mixins, supports extended features (like `icon` attribute) that require creating Shadow Root on it.
So generally, it would be great to allow it for any elements that do not have any user agent Shadow DOM currently.

Number of elements with user agent Shadow Root seems to be much smaller, so black list is a good way too. If new elements are added to specs - they can be either completely hackable with easily accessible and standardized Shadow Root or added to updated black list, whatever makes more sense.

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