a way to specify kwargs in `Callable`s and `ParamSpec`s

[DetachHead]

currently mypy supports it in mypy_extensions, so pyright needs an equivalent

from typing import *
from mypy_extensions import Arg

P = ParamSpec("P")

class A(Generic[P]): ...
a: A[[Arg(int, 'a')]]
c: Callable[[Arg(int, 'a')], None]

[KotlinIsland]

Can we just support mypy_extensions?