fix(typings): allow async listener in typed events

So that:

```ts
socket.on("my-event", async () => {
  // ...
});
```

is valid under the @typescript-eslint/no-misused-promises rule.

Related: #1486

lib/typed-events.ts

@@ -58,7 +58,7 @@ export type ReservedOrUserListener<
  * Needed because of https://github.com/microsoft/TypeScript/issues/41778
  */
 type FallbackToUntypedListener<T> = [T] extends [never]
-  ? (...args: any[]) => void
+  ? (...args: any[]) => void | Promise<void>
   : T;
 
 /**