Added repeat function with negative count check

Author: khalidbih

Sprint-3/2-practice-tdd/repeat.js

@@ -1,5 +1,9 @@
-function repeat() {
-  return "hellohellohello";
+function repeat(str, count) {
+  if (count < 0){
+    throw new Error("count cannot be negative");
+  }
+    
+  return str.repeat(count);
 }
 
 module.exports = repeat;