Add cart pricing module

src/format.ts

@@ -0,0 +1,7 @@
+export function formatCents(cents: number, currency = 'USD'): string {
+  return new Intl.NumberFormat('en-US', { style: 'currency', currency }).format(cents / 100)
+}
+
+export function formatLineItem(name: string, quantity: number, unitPriceCents: number): string {
+  return `${name} x${quantity} — ${formatCents(unitPriceCents * quantity)}`
+}