Fix create-app CLI lint failures in CI

Author: justin808

packages/create-react-on-rails-app/src/create-app.ts

@@ -226,7 +226,7 @@ export function validateAppName(name: string): { success: boolean; error?: strin
 export function createApp(appName: string, options: CliOptions): void {
   const appPath = path.resolve(process.cwd(), appName);
   const proRequested = options.pro || options.rsc;
-  let proModeLabel: string | null = null;
+  let proModeLabel: '--rsc' | '--pro' | null = null;
   if (options.rsc) {
     proModeLabel = '--rsc';
   } else if (options.pro) {