Expose Flexbox elements through GraphQL 4.

Closes #3426905

graphql/webform.extension.graphqls

@@ -6,6 +6,11 @@ extend type WebformElementWebformActions {
   submitLabel: String
 }
 
+extend type WebformElementWebformFlexbox {
+  alignItems: String
+  elements: [WebformElement]
+}
+
 extend type WebformElementTextarea {
   rows: Int
 }