Fix geocoder to new 3.x API (#151)

As index.html is using the latest version of leaflet-control-geocoder
and the API changed in 3.0.0 with this example:

```
Migrate from callbacks to Promise

-geocoder.geocode(query, callback);
+geocoder.geocode(query).then(callback);

-geocoder.reverse(latlng, scale, callback);
+geocoder.reverse(latlng, scale).then(callback);

Migrate from callbacks to async-await

-geocoder.geocode(query, callback);
+const results = await geocoder.geocode(query);

-geocoder.reverse(latlng, scale, callback);
+const results = await geocoder.reverse(latlng, scale);
```

The usage in the map widget also needs updating to correspond.