Add units to VehicleWheel3D suspension stiffness and damping

[aaronfranke]

Minor PR. In master VehicleWheel3D's suspension stiffness does not have its units documented, this PR fixes that.

The metric/SI unit for suspension stiffness is kg/s² or N/m. However, the values for typical cars are usually between 20,000 and 200,000, so it's common to use Mg/s² or N/mm instead to bring the range of values down to between 20 and 200. This is what Godot uses. The documentation already recommends values around 50, 100, and 200 depending on the type of car, and these are the values that produce sane results in testing.

Similarly, the metric/SI unit for suspension damping is kg/s or N⋅s/m. Typical values are in the hundreds, but Godot works with values between 0 and 1, so it is using Mg/s or N⋅s/mm instead.

Here is what it looks like in the inspector:

EDIT: I temporarily marked this PR as draft because 1) I looked at the code and did not see where any conversion factor of 1000 was taking place and 2) I realized that the cars in Truck Town may be misleading me due to their low mass (40kg vs a real car's 1000 to 2000 kg). However, I did some more testing, even with a car of higher mass more like a realistic car, values in N/mm produced much better results than trying to put in numbers in the tens of thousands. So I'm highly confident that N/mm is correct, even though I can't find where in the code this is determined.

[vlandemart]

Tiny changes, seem correct

[Calinou]

Looks good to me.

[akien-mga]

It seems way overkill to me to list units both in N and in kg·m/s² or combination thereof.

I think we should pick one standard and stick with it. AFAIK N is a SI unit, and from what I remember of my university time, this would typically be the only one people reason about, unless doing dimensional analysis - and I don't think it's our role to invite people to dimensional analysis in the unit suffix of the inspector.

So I would stick to only the N derivatives and not specify 2 sets of units for those already convoluted dimensions.

Unless it is commonly established to use Mg over N in those specific dimensions in car engineering, in which case I would suggest using only that and dropping the N equivalents.

[akien-mga]

Thanks!