Shopify Provider (#186)

* Start of Shopify provider

* add missing comment to documentation

* add missing env vars to documentation

* update reference to types file

framework/shopify/utils/storage.ts

@@ -0,0 +1,13 @@
+export const getCheckoutIdFromStorage = (token: string) => {
+  if (window && window.sessionStorage) {
+    return window.sessionStorage.getItem(token)
+  }
+
+  return null
+}
+
+export const setCheckoutIdInStorage = (token: string, id: string | number) => {
+  if (window && window.sessionStorage) {
+    return window.sessionStorage.setItem(token, id + '')
+  }
+}