[w3c/uievents] Define groups of events by the interaction that caused them (#311)

Currently we're attempting to specify interactionId in Event Timing: https://github.com/WICG/event-timing/pull/102.

It would be helpful if we could have a way to easily talk about groups of events that are associated because they correspond to some user interaction. In particular, we are interested in the following user interactions:
* Pressing (and potentially releasing) a key. This would cause at least `keydown`, `keypress`, and `keyup`. We care mostly about being able to group `keydown` and `keyup` as corresponding to the same physical key. I think this case is relatively simple: the `code` attribute values would be the same for such events. But obviously we don't want to group all keys with the same `code` in one group, but instead group the `keydown` either by itself or with the subsequent `keyup`, assuming there is no `keydown` with the same `code` in between the two.
* Taps and drags: this seems a bit trickier. We want to group `pointerdown`, `touchstart`, `pointerup`, `touchend`, `mousedown`, `mouseup`, and `click` caused by either a tap/click or a drag on the website. I think this is tricky for a couple of reasons. First, we do not want to include scrolls, which can trigger some of these events, like `pointerdown`. Second, I'm not quite sure if there is some analogue to `code` that uniquely identifies the physical trigger for all of those events. I know that MouseEvents have a `button` and PointerEvents have a `pointerId`, but can they be compared? We can probably ignore the touch events, and at the very minimum we want to group at least `pointerdown`, `pointerup`, and `click`, and as they are all PointerEvents then we can group them by `pointerId`. This similarly requires some logic to group only events that correspond to the same tap/drag together instead of just all events with the same `pointerId`.

Anyways, let me know if this makes sense and whether this is something you think could belong in this specification! I guess the second one might not since PointerEvents are in a separate spec?

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