[touch-events] Define the ontouch* event handler attributes (#11)

Specs need to define these explicitly, as not all events have the corresponding event handler attributes. See Fullscreen for an example: https://fullscreen.spec.whatwg.org/#api

Blink has `ontouchstart`, `ontouchend`, `ontouchmove` and `ontouchcancel` on the `Document`, `Element` and `Window` interfaces.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/touch-events/issues/11