Add mjs_bodyToFrame() to convert a body to a frame.

Also add test for attaching the world body.

PiperOrigin-RevId: 685603933
Change-Id: I3cfe7d226d6f032b5a587e44d0f4f18207977188

src/user/user_objects.h

@@ -331,6 +331,9 @@ class mjCBody : public mjCBody_, private mjsBody {
   // reset keyframe references for allowing self-attach
   void ForgetKeyframes() const;
 
+  // create a frame and move all contents of this body into it
+  mjCFrame* ToFrame();
+
   // get mocap position and quaternion
   mjtNum* mpos(const std::string& state_name);
   mjtNum* mquat(const std::string& state_name);