Add support for addEventListener(...) and other EventTarget methods

### Describe your environment

* Operating System version: NA
* Firebase SDK version: 4.2
* Firebase Product: messaging

### Describe the problem

Actual:

There's no `addEventListener()` method on the `firebase.messaging()` object.

Expected:

The following code outputs "MESSAGE 1" and "MESSAGE 2" when a message is received:

```js
firebase.messaging().addEventListener("message", () => console.log("MESSAGE 1"));
firebase.messaging().addEventListener("message", () => console.log("MESSAGE 2"));
```

Discussion:

The method names [`onMessage()`](https://firebase.google.com/docs/reference/js/firebase.messaging.Messaging#onMessage) and [`onTokenRefresh()`](https://firebase.google.com/docs/reference/js/firebase.messaging.Messaging#onTokenRefresh) suggest that `firebase.messaging.Messaging` implements the [EventTarget](https://developer.mozilla.org/en-US/docs/Web/API/EventTarget) interface. However, this is not the case, which impairs the DX, as well as making it impossible to easily add multiple listeners for the same event, remove listeners, etc.