fix(smithy): Union variant naming

[dnys1]

This was discovered as a bug when developing the config object. It's very easy to accidentally creating conflicting types due to how we name union variants. Consider the following Smithy model:

union AWSCategory {
  auth: AWSCategoryAuth
}

structure AWSCategoryAuth {}

This generates the following Dart code:

aws_category.dart (the AWSCategory union)

sealed class AWSCategory {
  const factory AWSCategory.auth() = AWSCategoryAuth;
}

final class AWSCategoryAuth {
  const AWSCategoryAuth();
}

aws_category_auth.dart (the AWSCategoryAuth structure)

abstract class AWSCategoryAuth {}

Two types with name AWSCategoryAuth are available in the global namespace which conflict. This is because the heuristic for union variant classes is '{unionName}{variantName}'.pascalCase. This is guaranteed to be unique amongst union variants but will conflict with a struct named the same.

Since this is a Dart-only limitation, it would not be appropriate to throw an error or not allow this. The $ symbol is frequently used to indicate generated code, so this is used to suffix all variant names so that they 1) never conflict with other symbols; and 2) make clear that these classes are Dart abstractions (they do not directly represent anything in the Smithy schema).

So the union file becomes:

sealed class AWSCategory {
  const factory AWSCategory.auth() = AWSCategoryAuth$;
}

final class AWSCategoryAuth$ {
  const AWSCategoryAuth();
}