added small exercise for jest testing

step2-04/demo/src/index.ts

@@ -0,0 +1,17 @@
+let counter = 0;
+
+export function getCount() {
+  return counter;
+}
+
+export function increment() {
+  return ++counter;
+}
+
+export function decrement() {
+  return --counter;
+}
+
+export function square(x: number) {
+  return x * x;
+}