feat: add simple Express app with /hello endpoint and tests

tests/unit/greeting.test.js

@@ -0,0 +1,7 @@
+const { getGreeting } = require('../../src/greeting');
+
+describe('getGreeting', () => {
+  it('returns the hello world message', () => {
+    expect(getGreeting()).toBe('Hello world');
+  });
+});