Additional PinSlotJoint 2D implementation with DOF: 1 translation + 1 rotation (#775)

* Additional groove joint 2D implementation with 1 translation + 1 rotation

* Conditional import for 2d feature flag

* Groove joint 2d tests in testbed examples

* Renamed GrooveJoint to PinSlotJoint

* cargo fmt

* Cross-reference to groove joint in Godot engine

examples2d/pin_slot_joint2.rs

@@ -0,0 +1,86 @@
+use rapier2d::prelude::*;
+use rapier_testbed2d::Testbed;
+
+pub fn init_world(testbed: &mut Testbed) {
+    /*
+     * World
+     */
+    let mut bodies = RigidBodySet::new();
+    let mut colliders = ColliderSet::new();
+    let mut impulse_joints = ImpulseJointSet::new();
+    let multibody_joints = MultibodyJointSet::new();
+
+    /*
+     * Ground
+     */
+    let ground_size = 3.0;
+    let ground_height = 0.1;
+
+    let rigid_body_floor = RigidBodyBuilder::fixed().translation(vector![0.0, -ground_height]);
+    let floor_handle = bodies.insert(rigid_body_floor);
+    let floor_collider = ColliderBuilder::cuboid(ground_size, ground_height);
+    colliders.insert_with_parent(floor_collider, floor_handle, &mut bodies);
+
+    /*
+     * Character we will control manually.
+     */
+    let rigid_body_character =
+        RigidBodyBuilder::kinematic_position_based().translation(vector![0.0, 0.3]);
+    let character_handle = bodies.insert(rigid_body_character);
+    let character_collider = ColliderBuilder::cuboid(0.15, 0.3);
+    colliders.insert_with_parent(character_collider, character_handle, &mut bodies);
+
+    /*
+     * Tethered cube.
+     */
+    let rad = 0.4;
+
+    let rigid_body_cube =
+        RigidBodyBuilder::new(RigidBodyType::Dynamic).translation(vector![1.0, 1.0]);
+    let cube_handle = bodies.insert(rigid_body_cube);
+    let cube_collider = ColliderBuilder::cuboid(rad, rad);
+    colliders.insert_with_parent(cube_collider, cube_handle, &mut bodies);
+
+    /*
+     * Rotation axis indicator ball.
+     */
+    let rigid_body_ball =
+        RigidBodyBuilder::new(RigidBodyType::Dynamic).translation(vector![1.0, 1.0]);
+    let ball_handle = bodies.insert(rigid_body_ball);
+    let ball_collider = ColliderBuilder::ball(0.1);
+    colliders.insert_with_parent(ball_collider, ball_handle, &mut bodies);
+
+    /*
+     * Fixed joint between rotation axis indicator and cube.
+     */
+    let fixed_joint = FixedJointBuilder::new()
+        .local_anchor1(point![0.0, 0.0])
+        .local_anchor2(point![0.0, -0.4])
+        .build();
+    impulse_joints.insert(cube_handle, ball_handle, fixed_joint, true);
+
+    /*
+     * Pin slot joint between cube and ground.
+     */
+    let axis: nalgebra::Unit<
+        nalgebra::Matrix<
+            f32,
+            nalgebra::Const<2>,
+            nalgebra::Const<1>,
+            nalgebra::ArrayStorage<f32, 2, 1>,
+        >,
+    > = UnitVector::new_normalize(vector![1.0, 1.0]);
+    let pin_slot_joint = PinSlotJointBuilder::new(axis)
+        .local_anchor1(point![2.0, 2.0])
+        .local_anchor2(point![0.0, 0.4])
+        .limits([-1.0, f32::INFINITY]) // Set the limits for the pin slot joint
+        .build();
+    impulse_joints.insert(character_handle, cube_handle, pin_slot_joint, true);
+
+    /*
+     * Set up the testbed.
+     */
+    testbed.set_world(bodies, colliders, impulse_joints, multibody_joints);
+    testbed.set_character_body(character_handle);
+    testbed.look_at(point![0.0, 1.0], 100.0);
+}