Support popover attributes

[fabiospampinato]

Example:

  interface VoidHTMLAttributes<T extends EventTarget> {
    popover?: FunctionMaybe<Nullable<string>>;
    ontoggle?: ObservableMaybe<Nullable<EventHandler<TargetedEvent<T, ToggleEvent>>>>;
    onbeforetoggle?: ObservableMaybe<Nullable<EventHandler<TargetedEvent<T, ToggleEvent>>>>;
  }