feat| create solution files

Author: nik-rev

13_factorial/factorial.js

@@ -0,0 +1,6 @@
+const factorial = function() {
+
+};
+
+// Do not edit below this line
+module.exports = factorial;

13_factorial/solution/factorial-solution.js

@@ -0,0 +1,8 @@
+const factorial = function(n) {
+  if (/[.-]/.match(n + '')) return;
+  if (n === 0 || n === 1) return 1;
+  return factorial(n - 1) + factorial(n - 2);
+};
+
+// Do not edit below this line
+module.exports = factorial;