Ammo slider constraint doesn't work

[diarmidmackenzie]

Ammo slider constraint, as used in the Ammo constraints example doesn't work at all.

comments in ammo-constraint suggest that the code to implement this has not been finished yet.

case CONSTRAINT.SLIDER: {
        //TODO: support setting linear and angular limits

Docs reference for btSliderConstraint is here:
https://pybullet.org/Bullet/BulletFull/classbtSliderConstraint.html

Fix-up probably will be similar to recent work done on Sprint constraint (about to be merged onto main).