feat: add simple inverse-kinematics solver for multibodies (#632)

* feat: add a simple jacobian-based inverse-kinematics implementation for multibodies

* feat: add 2d inverse kinematics example

* feat: make forward_kinematics auto-fix the root’s degrees of freedom

* feat: add 3d inverse kinematics example

* chore: update changelog

* chore: clippy fixes

* chore: more clippy fixes

* fix tests

src/pipeline/physics_pipeline.rs

@@ -469,10 +469,10 @@ impl PhysicsPipeline {
         // TODO: do this only on user-change.
         // TODO: do we want some kind of automatic inverse kinematics?
         for multibody in &mut multibody_joints.multibodies {
-            multibody.1.update_root_type(bodies);
-            // FIXME: what should we do here? We should not
-            //        rely on the next state here.
             multibody.1.forward_kinematics(bodies, true);
+            multibody
+                .1
+                .update_rigid_bodies_internal(bodies, true, false, false);
         }
 
         self.detect_collisions(