`ContainingClass` special form

this scenario is pretty niche, but i think it shows a missing capability in the type system:

```py
from basedtyping import ContainingClass

class MyFunction:
def __init__(self, fn: Callable[[ContainingClass], object]

class A:
a = MyFunction(lambda self: self.b)
# 'self' inferred as A/Self
b: int
```

additionally, this could be used to model the special cased behavior of `FunctionType`

Start by studying the ContainingClass example in the issue and the existing type-system handling for Callable and FunctionType. Determine how the proposed special form should infer self as A or Self, and define the expected behavior for the FunctionType case. Done means the capability is specified and both scenarios are covered.

Written by the indexing model from the issue text.