implement-is-oddlisted

# Implement isOdd (test-driven) Implement `isOdd(n)` in `src/isodd.js` as a named ESM export. It returns whether an integer is odd. Follow test-driven development, in this order: 1. **First** write a failing test in `test/isodd.test.js`. 2. **Then** implement `src/isodd.js` to make the test pass. Do not write the implementation before the test exists and fails.