add rule for riverpod code generation using riverpod annotation

[matthias-prevost]

Cursor tends to write provider without code generation.
This rule ensure it uses riverpod annotation, and use the Ref type instead of the deprecated <ProviderName>Ref

[giboin]

You don't need to give some why to the IA in this case, you just need some clear "do and don't" to make it understand what is good code and what isn't
So here you can just say The ref parameter of a provider is of type "Ref"

If you want to be more explicit, what you can do is giving an example of good code and an example of bad code in xml tags:

<examples>
  <good>
  
import 'package:flutter_riverpod/flutter_riverpod.dart';

@riverpod
Future<int> myProvider(Ref ref){...}

  </good>
  <bad>
  
import 'package:flutter_riverpod/flutter_riverpod.dart';

@riverpod
Future<int> myProvider(MyProviderRef ref){...}

  </bad>
</examples>

[giboin]

Always use the `@riverpod` annotation when writing providers.

It's more explicit in my opinion