It is useful for web authors do determine when their <dialog> elements open and close. popover already has `ToggleEvent` which is dispatched when a popover opens or closes, but <dialog> does not. The current way to detect when a <dialog> opens is to register a mutation observer to check for open, however, this is quite a lot of work where an event would be easier. This change incorporates the same `ToggleEvent`s that popovers dispatch, but for `<dialog>` elements: when `showModal` or `show` is called, <dialog> dispatches a ToggleEvent with newState=open. When a dialog is closed (via form or button or closewatcher) it should dispatch a ToggleEvent with newState=closed.