Add back missing solution to 03_reverseString

This was mistakenly removed when cherry picking the original file
rename commits, which include some errors and reversions.

See https://github.com/TheOdinProject/javascript-exercises/pull/325/files#r1092576896

03_reverseString/reverseString.js

@@ -1,5 +1,5 @@
-const reverseString = function(str) {
-
+const reverseString = function(string) {
+  return string.split('').reverse().join('');
 };
 
 module.exports = reverseString;