Add Triangle2d.arrowhead function?

[ianmackenzie]

Useful for 2D drawing but also potentially for 3D graphics (create in 2D then place in 3D). Something like:

Triangle2d.arrowhead :
    { tip : Point2d units coordinates
    , direction : Direction2d units coordinates
    , length : Quantity Float units
    , width : Quantity Float units
    }
    -> Triangle2d units coordinates

Should vertex order of the returned triangle be ( right, tip, left ) or ( tip, left, right )? ( right, tip, left ) is nicely symmetric and has the tip 'centered between' the left and right...