[display] detect unused type parameters

We should have an unused code diagnostic for type parameters, e.g:

```haxe
class Main {
static function main() {}
}
```

The issue names no source file, test, or entry point. Start by locating the compiler's display diagnostics and the handling of type parameters; done means the example reports an unused type-parameter diagnostic for T without flagging used type parameters.

Written by the indexing model from the issue text.