async function handleAddToCart(c) {
  const body = await c.req.parseBody();
  const { sku } = body;

  const items = readFromCookie(c);

  const lineItem = items.find(i => i.sku === sku);
  if (lineItem) {
    lineItem.quantity++;
  } else {
    items.push({ sku, quantity: 1 });
  }
  writeToCookie(items, c);
}
Handles adding an item to the cart.
@param {HonoContext} c - The hono context.