Provide public API for getting event listeners

Nobody has claimed this yet.

## Motivation

The API provides `on`, `once`, `off` for listeners, but no way of actually fetching them once they've been instantiated.
In our application code we add custom event listeners to the map, for example:

```javascript
this.map.on('levelChange', ({ level }: { level: number }) => {

});
```

Now without keeping track of this listener in some application state (which would be nice to avoid as we try and keep all map related state within the map object where possible), there doesn't seem to be a clean way to get hold of the listener using the public API. At the moment we work around that like this:

```javascript
this.map._listeners['levelChange']
```

To turn it on (and off).

## Design Alternatives

It would be great to be able to have a public API that allows for this.

## Design

The map would provide a method which allows the developer to get all listeners or one by name.

### Implementation

```
map.getListener('levelChange')
```

or/and:

```
map.getListeners()
```

Start with the public map event API (`on`, `once`, `off`) and the current `_listeners` access described in the issue. Clarify whether the API returns one listener or all listeners, then add the requested public access with tests demonstrating retrieval by event name and without a name.

Written by the indexing model from the issue text.