chore: Counter now embedded with Stimulus

Before this commit, Counter js was autonomous. Now it's registered
like a controller in Stimulus.

app/javascript/controllers/index.js

@@ -4,8 +4,8 @@
 
 import { application } from "./application"
 
-import HelloController from "./hello_controller"
 import ShadcnTestController from "./shadcn_test_controller"
+import CounterController from "./counter_controller"
 
-application.register("hello", HelloController)
 application.register("shadcn-test", ShadcnTestController)
+application.register("counter", CounterController)