Lets a component define its ARIA role through a `role` state member


Among other things, this allows a class or mixin to define a default role through the component's defaultState.

Some mixins come with identicial support for managing an ARIA role. Those mixins include AriaListMixin, AriaMenuMixin, DialogModalityMixin, and PopupModalityMixin. If you're using one of those mixins, you do not need to use this mixin.