[w3c/uievents] Clarify whether a click event should be dispatched for `control` + click in Mac OS (#265)

In Mac OS, mouse click with `control` key pressed will bring up the context menu, it is a platform convention, all browser (Gecko, Blink, Webkit) dispatch a `contextmenu` event followed a `mousedown` event for this case in Mac OS.

But whether to dispatch a `click` event is not consistent, test script: https://codepen.io/edgarchen-the-decoder/pen/ExVGRxB
- Webkit: Do send a `click` event for mouse click with `control` key pressed.
- Blink: No `click` event.
- Gecko: 
  - Release channel: No `click` event , it is because it dispatch `mousedown` and `mouseup` event with `button=2` instead.
  - Nightly channel: And we would like to change it to align with other browsers which dispatch events with `button=0`, see https://bugzilla.mozilla.org/show_bug.cgi?id=1615732. Then we dispatch a `click` event.

And we hit a regression on Google Sheets, see https://bugzilla.mozilla.org/show_bug.cgi?id=1638872. It looks like the Google Sheets depends on this behavior, and it has some special handle for Safari which will dispatch a `click` event for `control` + click.

And it looks reasonable to me to NOT dispatch a `click` event given that it already trigger a `contextmenu` event, it kinda conflicts with `click` event?

It will be good to define what is the behavior we would like to have for this platform convention. Thanks.

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