added tests to createPlugin, plus minor refactors

Co-authored-by: Patrik Oldsberg <poldsberg@gmail.com>
Co-authored-by: Johan Haals <johan.haals@gmail.com>
Signed-off-by: Fredrik Adelöw <freben@gmail.com>

packages/app-next/src/examples/graphiqlPlugin.tsx

@@ -29,5 +29,5 @@ export const GraphiqlPage = createPageExtension({
 
 export const graphiqlPlugin = createPlugin({
   id: 'graphiql',
-  defaultExtensions: [GraphiqlPage],
+  extensions: [GraphiqlPage],
 });