feat: Add callable type handling for stubs generator.

[Masara]

Is your feature request related to a problem?

Currently callable types are not handled during the stubs generation. Callable types should be handled according to the Safe-DS documentation.

Desired solution

From the Safe-DS documentation:

Safe-DS Stubs	Python Code
(a: Int, b: Int) -> r: Int	Callable[[int, int], int]
(a: Int, b: Int) -> (r: Int, s: Int)	Callable[[int, int], Tuple[int, int]]