[Feature Request] User-defined generics (type parameters for classes)

> Type parameters are only allowed in standard library

_But why?_

The request concerns extending type parameters from the standard library to user-defined classes. Start by locating the implementation and tests for existing standard-library generics, then determine the language and tooling changes needed for class type parameters. Done means user-defined generic classes are supported consistently with existing type parameters and covered by tests.

Written by the indexing model from the issue text.