Unused template parameter in model template

[chunyu3]

No diagnostic is issues for a templated model with a template parameter that is not used.

For example:

model Response<S, B> {
  @doc("The status code.")
  @statusCode
  _: S;
}

model Cat {
  meow: int32;
}

@route("/")
namespace root {
  op read(): Response<200, Cat>;
}

[chunyu3]

This issue is a sub-task of #181 which is a feature of typespec authoring in VSCode extension.