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/queries/get-collections-query.ts

@@ -0,0 +1,21 @@
+export const getCollectionsQuery = /* GraphQL */ `
+  query getCollections {
+    collections {
+      items {
+        id
+        name
+        description
+        slug
+        productVariants {
+          totalItems
+        }
+        parent {
+          id
+        }
+        children {
+          id
+        }
+      }
+    }
+  }
+`