Vendure provider (#223)

* Minimal list/detail views working with Vendure

* Implement useCart/useAddItem

* Implement useUpdateItem & useRemoveItem

* Implement useSearch

* Add operations codegen, tidy up

* Dummy checkout page

* Implement auth/customer hooks

* Use env var for Shop API url

* Add some documentation

* Improve error handling

* Optimize preview image size

* Fix accidental change

* Update Vendure provider to latest changes

* Vendure provider: split out gql operations, remove unused files

* Update Vendure provider readme

* Add local next.config to Vendure provider, update docs

* Update to use demo server

* Fix build errors

* Use proxy for vendure api

* Simplify instructions in Vendure readme

* Refactor Vendure checkout api handler

* Improve image quality

framework/vendure/cart/use-cart.tsx

@@ -0,0 +1,49 @@
+import { Cart } from '@commerce/types'
+import { SWRHook } from '@commerce/utils/types'
+import useCart, { FetchCartInput, UseCart } from '@commerce/cart/use-cart'
+import { ActiveOrderQuery, CartFragment } from '../schema'
+import { normalizeCart } from '../lib/normalize'
+import { useMemo } from 'react'
+import { getCartQuery } from '../lib/queries/get-cart-query'
+
+export type CartResult = {
+  activeOrder?: CartFragment
+  addItemToOrder?: CartFragment
+  adjustOrderLine?: CartFragment
+  removeOrderLine?: CartFragment
+}
+
+export default useCart as UseCart<typeof handler>
+
+export const handler: SWRHook<
+  Cart | null,
+  {},
+  FetchCartInput,
+  { isEmpty?: boolean }
+> = {
+  fetchOptions: {
+    query: getCartQuery,
+  },
+  async fetcher({ input: { cartId }, options, fetch }) {
+    const { activeOrder } = await fetch<ActiveOrderQuery>(options)
+    return activeOrder ? normalizeCart(activeOrder) : null
+  },
+  useHook: ({ useData }) => (input) => {
+    const response = useData({
+      swrOptions: { revalidateOnFocus: false, ...input?.swrOptions },
+    })
+
+    return useMemo(
+      () =>
+        Object.create(response, {
+          isEmpty: {
+            get() {
+              return (response.data?.lineItems.length ?? 0) <= 0
+            },
+            enumerable: true,
+          },
+        }),
+      [response]
+    )
+  },
+}