[red-knot] Add support for @classmethods (#16305)

## Summary

Add support for `@classmethod`s.

```py
class C:
    @classmethod
    def f(cls, x: int) -> str:
        return "a"

reveal_type(C.f(1))  # revealed: str
```

## Test Plan

New Markdown tests

crates/red_knot_python_semantic/resources/mdtest/subscript/tuple.md

@@ -70,7 +70,7 @@ def _(m: int, n: int):
 
     tuple_slice = t[m:n]
     # TODO: Support overloads... Should be `tuple[Literal[1, 'a', b"b"] | None, ...]`
-    reveal_type(tuple_slice)  # revealed: @Todo(return type)
+    reveal_type(tuple_slice)  # revealed: @Todo(return type of decorated function)
 ```
 
 ## Inheritance