add missing typescript declaration of Vector2.cross

src/math/Vector2.d.ts

@@ -305,6 +305,11 @@ export class Vector2 implements Vector {
    */
   dot(v: Vector2): number;
 
+  /**
+   * Computes cross product of this vector and v.
+   */
+  cross(v: Vector2): number;
+
   /**
    * Computes squared length of this vector.
    */