support additional markers

[frankely]

Scenario: When creating the MapLocationPicker, I would like to specify my own additional markers

So they can be loaded in _MapLocationPickerState.build

    const additionalMarkers = widget.additionalMarkers.map(entry => {
      return Marker(
        markerId: MarkerId(entry.key),
        position: entry.value,
      );
    }).toList();

    final markers = Set<Marker>.from(additionalMarkers);
    markers.add(Marker(
      markerId: const MarkerId("one"),
      position: _initialPosition,
    ));

//...
    markers: markers,
//...

[rvndsngwn]

https://github.com/rvndsngwn/map_location_picker/pull/9 Merged.