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/fetcher.ts

@@ -0,0 +1,51 @@
+import { Fetcher } from '@commerce/utils/types'
+import { FetcherError } from '@commerce/utils/errors'
+
+async function getText(res: Response) {
+  try {
+    return (await res.text()) || res.statusText
+  } catch (error) {
+    return res.statusText
+  }
+}
+
+async function getError(res: Response) {
+  if (res.headers.get('Content-Type')?.includes('application/json')) {
+    const data = await res.json()
+    return new FetcherError({ errors: data.errors, status: res.status })
+  }
+  return new FetcherError({ message: await getText(res), status: res.status })
+}
+
+export const fetcher: Fetcher = async ({
+  url,
+  method = 'POST',
+  variables,
+  query,
+  body: bodyObj,
+}) => {
+  const shopApiUrl =
+    process.env.NEXT_PUBLIC_VENDURE_LOCAL_URL ||
+    process.env.NEXT_PUBLIC_VENDURE_SHOP_API_URL
+  if (!shopApiUrl) {
+    throw new Error(
+      'The Vendure Shop API url has not been provided. Please define NEXT_PUBLIC_VENDURE_SHOP_API_URL in .env.local'
+    )
+  }
+  const hasBody = Boolean(variables || query)
+  const body = hasBody ? JSON.stringify({ query, variables }) : undefined
+  const headers = hasBody ? { 'Content-Type': 'application/json' } : undefined
+  const res = await fetch(shopApiUrl, {
+    method,
+    body,
+    headers,
+    credentials: 'include',
+  })
+
+  if (res.ok) {
+    const { data } = await res.json()
+    return data
+  }
+
+  throw await getError(res)
+}