feat: Convert all app files to JS

packages/backend/src/apps/deepl/common/add-auth-header.js

@@ -0,0 +1,10 @@
+const addAuthHeader = ($, requestConfig) => {
+  if ($.auth.data?.authenticationKey) {
+    const authorizationHeader = `DeepL-Auth-Key ${$.auth.data.authenticationKey}`;
+    requestConfig.headers.Authorization = authorizationHeader;
+  }
+
+  return requestConfig;
+};
+
+export default addAuthHeader;