Add a `lib` mode that is 484 compatable

lib authors won't be able to use this at all if it has breaking changes, so add a `lib` mode that is compatible with mypy.

Honestly this sounds very unbased.

Start by reviewing the existing mode or configuration entry points and the project's compatibility expectations for mypy and PEP 484. The issue names no files or tests and does not define the `lib` mode's behavior, so clarify its scope before implementation. Done should mean that library authors can use the mode without the breaking changes described in the issue.

Written by the indexing model from the issue text.