Disallow access of private members

[MartinBernstorff]

I'm not deep enough in type theory to know if this is related, but I'd love if pyrefly had an option to enforce that you cannot access private members, e.g.:

class MyClass:
    _private_field: int

    def _private_method(self) -> None:
        pass


MyClass()._private_field # Should error
MyClass()._private_method() # Should error

Originally posted by @MartinBernstorff in #1126

[jorenham]

Feels more like a feature that'd be more appropriate for a linter than a type-typechecker TBH.