PhysicsStepListener: add the onStep() method

src/main/java/com/github/stephengold/joltjni/PhysicsStepListener.java

@@ -29,5 +29,14 @@ of this software and associated documentation files (the "Software"), to deal
  * @author Stephen Gold [email protected]
  */
 public interface PhysicsStepListener extends ConstJoltPhysicsObject {
+    // *************************************************************************
+    // new methods exposed
 
+    /**
+     * Callback invoked (by native code) each time the system is stepped.
+     *
+     * @param contextVa the virtual address of the
+     * {@code PhysicsStepListenerContext} (not zero)
+     */
+    void onStep(long contextVa);
 }