Suggested controller: SlotController

Within the community, various teams already have some form of a slot controller to basically abstract the slotchanged/assigned elements logic. It'd probably be a good idea for us to provide such a controller. It could also act as a base to other slot-based controllers. basically: ```ts class SlotController implements ReactiveController { constructor( host: ReactiveControllerHost, slotRef: Ref, selector?: string ); } ``` how we pass in a change handler, im not sure yet. could be a constructor arg, or a method, etc.