[V3 Todo] Change default icon positioning to match leaflet icon class

[adamscybot]

At the moment, the Icons use the DivIcon default which is for the anchor point to be on the centre (both x and y) of the icon. But the user doesn't care about the implications of the DivIcon abstraction.

We want to instead position the icon by default such that the anchor is considered bottom center (which works with basic symmetrical "pin" icons).

At the moment the user has to tweak the positioning of the icon which is not ideal and could also be considered a bug.