[IMP] runtime: improve useExternalListener typing

Previously the type of the target for useExternalListener was
HTMLElement or Window, this makes document an invalid target. Here there
is no reason to use the EventTarget interface instead, as
useExternalListener only uses methods from that interface and should
work with any event target.

Closes odoo/owl#1323

src/runtime/hooks.ts

@@ -117,7 +117,7 @@ export function useEffect(effect: Effect, computeDependencies: () => any[] = ()
  *  `useExternalListener(window, 'click', this._doSomething);`
  * */
 export function useExternalListener(
-  target: HTMLElement | typeof window,
+  target: EventTarget,
   eventName: string,
   handler: EventListener,
   eventParams?: AddEventListenerOptions