Diagnostic: Calling application.register with a non-controller should warn you

[marcoroth]

If you try to register something the Stimulus Parser cannot trace to a controller definition we should add a diagnostic to the application.register() call.

// app/javascript/controllers/index.js

import { application } from "./application"

import User from "./user_controller"

application.register("user", User)
/*                           ^^^^ 
┌──────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ The argument `User` couldn't be resolved to a Stimulus controller definition. Make sure you are      │
│ passing in a reference to a valid Stimulus controller definition.                                    │
│ Stimulus LSP (stimulus.controller.value.type.invalid)                                                │
├──────────────────────────────────────────────────────────────────────────────────────────────────────┤
│  View Problem (⌥F8)   Quick-Fix... (⌘.)                                                              │
└──────────────────────────────────────────────────────────────────────────────────────────────────────┘
*/