Add tax calculation

src/tax.ts

@@ -0,0 +1,10 @@
+import { totalCents, type CartItem, type Discount } from './cart'
+
+export function totalWithTaxCents(
+  items: CartItem[],
+  taxRatePercent: number,
+  discount?: Discount,
+): number {
+  const base = totalCents(items, discount)
+  return base + base * (taxRatePercent / 100)
+}