Standardize `event.which` for MouseEvent

[scottgonzalez]

Browsers implement event.which for mouse events, and many users rely on it, but it's not defined in DOM Events. There is a standard for event.button and event.buttons, but according to jQuery's source it seems like at least one browser treats event.button as event.buttons.

[leobalter]

This is being tracked at w3c/uievents#35

It seems this is becoming part of the legacy UIEvent interface. https://w3c.github.io/uievents/#dom-keyboardevent-which

@scottgonzalez can we close this or do you prefer waiting that uievents issue to be done?

[scottgonzalez]

I think ultimately we'll say we don't care about Mouse Events and just say this is solved with Pointer Events. So I'm ok with closing this since button and buttons should be correct in the new implementations.