2d version should be foremost in implementing a 2d concept (added `two_d` to involute_gear.scad `gear()`)

Currently if the user wants to `linear_extrude` the involute gear he has to figure out the arguments of `gear_shape`, there is no good reason for this.

The use of `if` for this is a bit bad imo, problem is that passing the arguments is a pita.