fix: move app to src

Author: thomasrockhu-codecov

app/calculator.js renamed to src/calculator.js

@@ -37,5 +37,5 @@ test('divide function', () => {
   expect(divide(1.0, 2.0)).toBe(0.5);
   expect(divide(0, 2.0)).toBe(0);
   expect(divide(-4, 2.0)).toBe(-2.0);
-  // expect(divide(2.0, 0)).toBe('Cannot divide by 0');
+  expect(divide(2.0, 0)).toBe('Cannot divide by 0');
 });