Hook up joint_limit_kd and joint_limit_ke to jnt_solref

[adenzler-nvidia]

Also looking at this PR: https://github.com/newton-physics/newton/pull/710/files where we always convert solref to stiffness/damping.

We can just use the negative convention and set jnt_solref from limit_ke/kd. This also means we can remove the explicit default handling in the mujoco solver.

This should include:

• correct solref setup during mjmodel creation
• expansion to multiple worlds
• a hook into the solver notification system to change this at runtime
• testing.

I'm assuming some tests will fail and this might have implications on the benchmarks and IsaacLab. We should study this to figure out where we need to set this explicitly or whether it's ok to leave at default values