Added alignment property

[codeOfJannik]

Adding the alignment property to LocationMarkerStyle to set the alignment property on the Marker class of flutter_map: https://pub.dev/documentation/flutter_map/latest/flutter_map/Marker/alignment.html

Alignment of the marker relative to the normal center at point For example, Alignment.topCenter will mean the entire marker widget is located above the point.

The center of rotation (anchor) will be opposite this.

Defaults to Alignment.center if also unset by MarkerLayer.