[css-variables] Event when property value changes

[ByteEater-pl]

It would be useful to have an event dispatched to a (pseudo)element when its CSS property (any, actually, but custom ones seem to be a good first step) changes.

Relevant section

[tabatkins]

Agreed that this would be useful, but it's likely part of a larger effort; at minimum, it's more than level 1 of this spec wants to handle. I'll punt this to level 2. It might go somewhere else eventually, but that's at least a good place to park it.