Add reference implementation of PEP 718

I think this is very much needed PEP, and using mypy for reference implementation is a good idea because mypy actually already (mostly) supports this under the hood.

The only non-trivial question is whether we should allow type applications only to references that are statically known to refer to a function, or do we want to support type application to an arbitrary expression that happen to have a callable-like type inferred? For example:

T = TypeVar("T")

class Callbacks(TypedDict):
    regular: Callable[[int], int]
    generic: Callable[[T], T]

def test(cb: Callbacks) -> None:
    cb["generic"][int]  # <- should we support this?

If we only want to support static references, then implementation is trivial, essentially:

• Make this error conditional on Python version <3.15 and file being not a stub.
• Update this helper to accept a Decorator.

However, if we want to support arbitrary expressions, it should be quite easy as well. Luckily, all the necessary infrastructure was added for TypeForm PEP support. We can use TypeCheckerAsSemanticAnalyzer adapter to re-interpret arbitrary expression as type during type checking.

cc @JukkaL @Gobot1234 @PabloRuizCuevas

Start by reading the linked checks in mypy/checkexpr.py around lines 4948-4949 and the helper in mypy/semanal.py around line 8235, then compare them with PEP 718 and the existing TypeForm infrastructure. The work is done when the project has an agreed reference implementation and its supported type-application cases and Python-version behavior are covered by tests.