Generalize AccessibilityRole/AriaAttributes IDL (#984)

* Generalize AccessibilityRole/AriaAttributes IDL

This generalizes the AccessibilityRole/AriaAttributes IDL mixins to allow them to have different behaviors per host interface. For Element, they have the reflection behavior specified, but e.g. for ElementInternals, HTML can use this framework to define different behavior.

In the process, this consolidates the two mixins into one ("ARIAMixin"), since this makes it easier to define their behavior uniformly, and consumers should generally not mix in one without the other.

Author: domenic