STL format: Quick select target unit system

STL files are unit less and therefore are assumed to be in mm by most programs like slicers.

Blender uses the metric system by default and therefore a lot of beginners get confused when their 2m default cube is actually only 2mm in most programs and asume blender is bugged.

Usually this can be accounted for by setting the scale factor in the export settings.

I suggest having a new dropdown added which indicates the target unit system (e.g. mm for most slicer programs) and thus sets the scale factor accordingly automatically. Could then also be the default setting of the stl format addon.