[touch-events] should TouchEvent define getModifierState (#41)

It seems odd to define the alt, meta, ctrl, shift keys on Touch Events; but nonetheless they are there.

We should define the getModifierState defined in https://w3c.github.io/uievents/#widl-KeyboardEvent-getModifierState if we do https://github.com/w3c/touch-events/issues/40 

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https://github.com/w3c/touch-events/issues/41