Add typing back to actx.author

[krittick]

Summary

Redo of #464, which was reverted without explanation in #848

After reflection, this new PR handles it in a slightly more appropriate way instead of defining an entirely new cached_property for what's effectively an alias from author to user.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, typehinting, examples, ...)