Added handlers for the cart API

lib/bigcommerce/api/utils/parse-item.ts

@@ -0,0 +1,9 @@
+import type { ItemBody } from '../cart'
+
+const parseItem = (item: ItemBody) => ({
+  quantity: item.quantity,
+  product_id: item.productId,
+  variant_id: item.variantId,
+})
+
+export default parseItem