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-actions.tsx

@@ -0,0 +1,13 @@
+import useAddItem from './use-add-item'
+import useRemoveItem from './use-remove-item'
+import useUpdateItem from './use-update-item'
+
+// This hook is probably not going to be used, but it's here
+// to show how a commerce should be structuring it
+export default function useCartActions() {
+  const addItem = useAddItem()
+  const updateItem = useUpdateItem()
+  const removeItem = useRemoveItem()
+
+  return { addItem, updateItem, removeItem }
+}