support a `notabstract` decorator

```py
class A(ABC):
@abstractmethod
@classmethod
def foo(): ...

@notabstract
@classmethod
def bar(cls):
print("hi")
A.foo() # should be an error
A.bar() # should be okay
```

maybe this could work for instances too, idk

related:
- #752

Start by reviewing related issue #752 and tracing how basedmypy handles abstractmethod and classmethod decorators. Completion would require a defined `notabstract` behavior for the shown class methods, with `A.foo()` rejected and `A.bar()` accepted; instance behavior remains unspecified.

Written by the indexing model from the issue text.