feat: add URDF and STL importers

[sebcrozet]

This PR introduces two new crates:

• rapier3d-urdf for loading URDF files into rapier3d. This will load the rigid-bodies, colliders, and joints.
• rapier3d-stl for loading an STL file as a collision shape.

Added

• Add Collider::converted_trimesh and MeshConverter for building a collider with a shape computed from a mesh’s
  index and vertex buffers. That computed shape can currently be a TriMesh, a Cuboid (covering the mesh’s AABB or
  OBB), a convex hull, or a convex decomposition.
• Implement Default for RigidBodyBuilder. This is equivalent to RigidBodyBuilder::dynamic().
• Implement Default for ColliderBuilder. This is equivalent to ColliderBuilder::ball(0.5).

Modified

• Renamed JointAxesMask::X/Y/Z to ::LIN_X/LIN_Y/LIN_Z; and renamed JointAxisMask::X/Y/Z to ::LinX/LinY/LynZ to
  make it clear it is not to be used as angular axes (the angular axis are JointAxesMask::ANG_X/ANG_Y/AngZ and
  JointAxisMask::AngX/AngY/AngZ).