Added enableDebug FX

[eelcoj]

This PR adds a enableDebug FX. It will provide/provides extended debugging capabilities to Stimulus, by listing your values and targets.

Usage

// app/javascript/controllers/application.js
+import { enableDebug } from "stimulus-fx"

-const application = Application.start()
+const application = enableDebug(Application.start())

Then enable per controller:

 export default class extends Controller {
+  static debug = true
+
  // …
}

TODO

• Extend README (need separate sections for FX now)