feat(mapvis): implement directed arrows

This took a lot longer than expected since we have to deal with projections from mercator back to pixel world space. Theres a lot of linear algebra involved to calculate the paths, but it works now 😄. Note that at the moment, the arrows do not size with zoom: you have to zoom in quite a bit to see the arrowheads. I chose this because in low zoom levels, the many arrow heads massively overlap, and its impossible to tell which arrowhead belongs to which edge.

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