Add implicit viscous joint damping

[hsu]

Support viscous joint damping using a constraint on the joint's DOF with zero ERP and finite CFM. References:

• Conversion functions KpKdToCFMERP or CFMERPToKpKd
• joint damping test using this world

[scpeters]

One tricky aspect for us was switching the joint damping constraint on and off at joint limits (see ODEJoint::ApplyImplicitStiffnessDamping).

[erwincoumans]

I won't have time to implement the feature for Bullet 2.83. I wonder how your damping (and spring) compares with the btGeneric6DofSpring2Constraint version?