fix(math): modf doesn't have second return value type (#10)

Author: martinjlowm

core/math.d.ts

@@ -77,8 +77,9 @@ declare namespace math {
   /**
    * Returns the integral part of x and the fractional part of x. Its second
    * result is always a float.
+   * @tupleReturn
    */
-  function modf(x: number): number;
+  function modf(x: number): [number, number];
 
   /**
    * The value of π.