Add Pow.js (#498)

* Add pow.js

* Add tests for Pow.js

Maths/Pow.js

@@ -0,0 +1,11 @@
+// Returns the value of x to the power of y
+
+const pow = (x, y) => {
+  let result = 1
+  for (let i = 1; i <= y; i++) {
+    result *= x
+  }
+  return result
+}
+
+export { pow }