Add delta arg to WORLD_STEP event

src/physics/arcade/World.js

@@ -983,7 +983,7 @@ var World = new Class({
                 }
             }
 
-            this.emit(Events.WORLD_STEP);
+            this.emit(Events.WORLD_STEP, fixedDelta);
         }
 
         //  Process any additional steps this frame
@@ -1042,7 +1042,7 @@ var World = new Class({
             }
         }
 
-        this.emit(Events.WORLD_STEP);
+        this.emit(Events.WORLD_STEP, delta);
 
         this.stepsLastFrame++;
     },

src/physics/arcade/events/WORLD_STEP_EVENT.js

@@ -16,5 +16,7 @@
  *
  * @event Phaser.Physics.Arcade.Events#WORLD_STEP
  * @since 3.18.0
+ *
+ * @param {number} delta - The delta time amount of this step, in seconds.
  */
 module.exports = 'worldstep';