Migrate resource from flutter_map_dragmarker library

[RuyVictor]

Merge the flutter_map_dragmarker library for markers.
In the marker constructor, a canDrag boolean must be included to define whether it is a marker that allows dragging or not. Merging the library also allows you to avoid redundant layers, such as DragMarkers & MarkerLayer, consequently reducing boilerplate.

Disclaimer: There is no way to refute it as it is a notable problem that needs to be solved, do not consider this as a suggestion

[RuyVictor]

Here's an example of why it sucks

[ibrierley]

Author here. I don't have a particular problem if this was wanted to happen. Typically the issue is one of extra load on other devs to maintain other peoples code though, but I would try and support any problems related where possible.