stubtest: Support `type_check_only` decorator

Feature

I would like stubtest to support checking the type_check_only decorator.
This means something marked with type_check_only that DOES exist at runtime is an error.
And something that DOES NOT exist at runtime, and is marked with type_check_only should pass.

Pitch

Instead of marking stub-only classes and methods as private in typeshed, they can be marked with @type_check_only, which is designed for that exact purpose. Private may or may not be found at runtime, and users sometimes don't have a choice to use them, and basically have to guess whether they need to wrap the import in TYPE_CHECK_ONLY or not.
This would also promote marking as type_check_only special methods that don't exist at runtime, but are added to support some typechecker features, in typeshed.

Related: https://github.com/python/mypy/issues/9531

Inizia tracciando il punto di ingresso di stubtest e la gestione esistente dei decoratori e dei controlli di presenza a runtime. Confronta il comportamento richiesto per @type_check_only con i test attuali di stubtest e l’utilizzo in typeshed. Il lavoro è completato quando vengono segnalate le dichiarazioni type_check_only presenti a runtime, mentre quelle assenti passano, con copertura per entrambi i casi.