Delete CodeController.onChange

[alexeyinkin]

CodeController.onChange can be replaced with

controller.addListener(() {
  final text = controller.rawText;
});

If we delete onChange(), the benefits are:

• User code is less messy. If they use both ways to listen to changes, their code is hard to maintain.
• Less code for us to maintain.
• Following Flutter convention. Controllers are listened to, and widgets have onChanged because they are disposable and so cannot be listened to. Both TextField and CodeField have one.
• More consistent API. CodeField.onChanged is called with dots with webSpaceFix while CodeController.onChange is called with spaces.

@BertrandBev still you added onChange after rawText, so I wander what was the motivation for it. Are you OK with deleting it in the next breaking release?