add-cart-discountlisted

<!-- vigiles:result "npm test" --> # Add per-item discounts `total(items)` in `src/cart.js` sums `price * qty` across the cart. Add support for an optional `discount` field on an item — a percentage off that item's line total. For example an item `{ price: 100, qty: 1, discount: 10 }` contributes `90`. Items without a `discount` field are unaffected. The behaviour is covered by the tests under `test/`.