feat: Convert all app files to JS

packages/backend/src/apps/deepl/auth/verify-credentials.js

@@ -0,0 +1,9 @@
+const verifyCredentials = async ($) => {
+  await $.http.get('/v2/usage');
+
+  await $.auth.set({
+    screenName: $.auth.data.screenName,
+  });
+};
+
+export default verifyCredentials;