Drag event types support added

[GULERTOLGA]

onFeatureDrag was fired only when dragging the feature. Along with this PR, onFeatureDrag is also fired at the start and end of the drag. The event type is added to the payload with an enumeration.

  void _onMapCreated(MapboxMapController controller) {
    this.controller = controller;
    controller.onFillTapped.add(_onFillTapped);
    this.controller!.onFeatureDrag.add(_onFeatureDrag);
  }

  void _onFeatureDrag(id,
      {required current, required delta, required origin, required point, required eventType}) {
    DragEventType type = eventType;
    switch (type) {
      case DragEventType.start:
        // TODO: Handle this case.
        break;
      case DragEventType.drag:
        // TODO: Handle this case.
        break;
      case DragEventType.end:
        // TODO: Handle this case.
        break;
    }
  }

[felix-ht]

please also run dart format

[felix-ht]

LGTM