Linting rule against non model response

Copied from https://github.com/Azure/typespec-azure-pr/issues/2600

```
op world(): MyEnum;
```
returns a string. It should be disallowed.

General request is: If the return type is not a model, then it should be disallowed.

> I'd be fine with a rule that flags a non-model return type for application/json.
> from @mikekistler

No implementation files or tests are named. Start by locating the existing lint-rule entry points and tests for operation return types, then determine how application/json responses are represented. Done means the rule flags operations returning a non-model type, such as MyEnum, while the expected model-returning cases remain valid.

Written by the indexing model from the issue text.