problems(js): add “2620. Counter”

Signed-off-by: Matej Focko <mfocko@redhat.com>

problems/counter.js

@@ -0,0 +1,16 @@
+/**
+ * @param {number} n
+ * @return {Function} counter
+ */
+var createCounter = function(n) {
+    return function() {
+        return n++;
+    };
+};
+
+/**
+ * const counter = createCounter(10)
+ * counter() // 10
+ * counter() // 11
+ * counter() // 12
+ */