TYP: fix a few annotations in offsets.pyi
- [ ] closes #46908
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).

Some doc-strings refer to `pd.HolidayCalendar` but that doesn't seem to exist?!

There are a few more un-annotated variables and a few containers (list and dict) without types but I'm not familair enough with them to annotate them.
> Have we standardized on whether we put `None` last or first?

Not that I'm aware of. The only example I found is here: https://pandas.pydata.org/docs/development/contributing_codebase.html#style-guidelines (None at the end).

 > > Have we standardized on whether we put `None` last or first?
> 
> Not that I'm aware of. The only example I found is here: https://pandas.pydata.org/docs/development/contributing_codebase.html#style-guidelines (None at the end).

I think we use the style of `str | None` more than `None | str`, so can you use the former throughout in this file for this PR? Thanks @twoertwein 