Add decorator to mark class as concrete / instantiable

We have the @final decorator to mark a class as final, so that it cannot be subclassed. Typecheckers can then cue off of this to raise an error when there are unimplemented abstract methods (e.g. the Ty abstract-method-in-final-class rule).

Similarly, Pyright and Ty can flag errors when other code instantiates a class that has abstract methods.

However, I would find it very useful to be able to mark a class as concrete: it isn't final (someone might subclass it), but it is intended to be instantiable. This would allow the type checker to raise abstract method errors at the point of class definition, rather than elsewhere in the code where the class is instantiated. For classes that are instantiated directly, the error location isn't a huge problem; for classes that are primarily instantiated indirectly, however, it there is no guarantee that the code will include a direct instantiation of the class to raise an error.

Empieza revisando la propuesta junto con el decorador @final existente y el comportamiento de los métodos abstractos descrito para Pyright y Ty. Define la semántica prevista de comprobación de tipos del decorador de clases concretas y confirma que los errores se notifican en el momento de definir la clase para las clases con métodos abstractos, sin hacer que la clase sea final.