Adding density for surface electric currents (2D currents), [ElectricCurrent / Length].
Not sure what is the best name for the quantity:

• SurfaceElectricCurrentDensity (proposed in this PR, "surface current density" is typical in literature)
• ElectricCurrentSurfaceDensity (Easier to find in alphabetically sorted list of quantities, but sounds misleading, not specifying the 2D nature of current)
• ElectricSurfaceCurrentDensity (Also easier to find, stresses the 2D nature of the current, but just sound weird for me)