group stimulus scripts in an asset subfolder

app/assets/app.js

@@ -9,4 +9,4 @@
 import './styles/app.css';
 
 // start the Stimulus application
-import './bootstrap';
+import './stimulus/bootstrap';

app/assets/stimulus/controllers/hello_controller.js

@@ -11,6 +11,6 @@ import { Controller } from '@hotwired/stimulus';
  */
 export default class extends Controller {
     connect() {
-        this.element.textContent = 'Hello Stimulus! Edit me in assets/controllers/hello_controller.js';
+        this.element.textContent = 'Hello Stimulus! Edit me in assets/stimulus/controllers/hello_controller.js';
     }
 }