Add stub for `isZero`.

stubs/internal/Base.php

@@ -141,6 +141,16 @@ interface Base extends Countable
      */
     function isFinite();
 
+    /**
+     * Checks whether all elements of the object are zero (value <= tolerance).
+     * 
+     * For objects with complex numbers, real and imaginary part are checked separately.
+     * 
+     * @param number $tol[optional]
+     * @return bool
+     */
+    function isZero($tol);
+
     /**
      * Checks whether at least one of the elements of the object is ±infinity.
      *