Updated gravity definition. (#13088)

matter-js/matter-js.d.ts

@@ -2924,10 +2924,14 @@ declare namespace Matter {
     }
 
     interface IWorldDefinition extends ICompositeDefinition {
-        gravity?: Vector;
+        gravity?: Gravity;
         bounds?: Bounds;
     }
 
+    interface Gravity extends Vector {
+      scale: number;
+    }
+
     /**
     * The `Matter.World` module contains methods for creating and manipulating the world composite.
     * A `Matter.World` is a `Matter.Composite` body, which is a collection of `Matter.Body`, `Matter.Constraint` and other `Matter.Composite`.
@@ -2994,7 +2998,7 @@ declare namespace Matter {
          */
         static create(options: IWorldDefinition): World;
 
-        gravity: Vector;
+        gravity: Gravity;
         bounds: Bounds;
 
     }