add immutable var class

[adhami3310]

The ImmutableVar is a dataclass that inherits from Var, very similar to the current BaseVar, except without any setter or default value logic and with frozen=True so that it is immutable.

[masenf]

needs an overridden _replace method that returns ImmutableVar instances.

it actually might make sense to override create as well to something like this

@classmethod
def create(self, ...):
    return super().create(...)._replace()

That way we can create ImmutableVar instances in the same way as Var instances until more special-case behavior is added.

override create in last commit

[masenf]

we can hold off on adding tests for now, because most of the API for ImmutableVar is coming from Var itself and won't represent the final usage of the API.

but in a near-future PR, we will want to make sure these changes are coming in with near-full test coverage, because it's much easier to add tests incrementally, rather than at the end.

changes addressed