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/lib/mutations/adjust-order-line-mutation.ts

@@ -0,0 +1,15 @@
+import { cartFragment } from '../fragments/cart-fragment'
+
+export const adjustOrderLineMutation = /* GraphQL */ `
+  mutation adjustOrderLine($orderLineId: ID!, $quantity: Int!) {
+    adjustOrderLine(orderLineId: $orderLineId, quantity: $quantity) {
+      __typename
+      ...Cart
+      ... on ErrorResult {
+        errorCode
+        message
+      }
+    }
+  }
+  ${cartFragment}
+`