Type stubs for envparse

This package adds type annotations for the envparse library.

Known Issues

The force and preprocessor parameters have been omitted.
A default value of None is not allowed for list(), set(), tuple() or dict(). Instead, use an empty list/set/tuple/dict as the default.
A schema cannot be provided to Env.__init__(), as this would make it impossible to infer the return types of other methods.
No type hints for Env.__call__() yet.