refactor: rewrite function using es6 for consistency

striker-ui/lib/fetchers/fetchJSON.ts

@@ -1,5 +1,5 @@
-function fetchJSON<T>(...args: [RequestInfo, RequestInit?]): Promise<T> {
+const fetchJSON = <T>(...args: [RequestInfo, RequestInit?]): Promise<T> => {
   return fetch(...args).then((response: Response) => response.json());
-}
+};
 
 export default fetchJSON;