Add index.html with Stimulus hello controller

assets/js/app.js

@@ -0,0 +1,5 @@
+import { Application } from "@hotwired/stimulus";
+import { definitions } from "stimulus:./controllers";
+
+const app = Application.start();
+app.load(definitions);

assets/js/controllers/hello_controller.js

@@ -0,0 +1,7 @@
+import { Controller } from "@hotwired/stimulus";
+
+export default class extends Controller {
+  connect() {
+    console.log("hello#connect");
+  }
+}