[FEATURE] Publish PEP 561 types

[candrews]

Is your feature request related to a problem? Please describe.
It would be really nice if pyaml_env could publish PEP 561 type information allowing users of pyaml_env to use mypy to type check their code.

Thank you!

Describe the solution you'd like
Either include type information in this library so type checkers (like mypi) can use it, or publish a stub library with that information.

Describe alternatives you've considered
I'm currently ignoring pyaml_env entirely for type checking purposes as described in the documentation at https://mypy.readthedocs.io/en/stable/common_issues.html#spurious-errors-and-locally-silencing-the-checker

import pyaml_env  # type: ignore