Diagnostic: Warn if controller extends `Controller` from `stimulus` package

[marcoroth]

We should add a diagnostic to "warn" people that they should migrate the the "new" package in the @hotwired namespace.

Bad:

import { Controller } from "stimulus"

export default class extends Controller { 
  // ...
}

Good:

import { Controller } from "@hotwired/stimulus"

export default class extends Controller { 
  // ...
}

---

Bad:

import { Application } from "stimulus"

const application = Application.start()

Good:

import { Application } from "@hotwired/stimulus"

const application = Application.start()