COMPAT: alias .to_numpy() for scalars
``Timestamp`` and ``Timedelta`` scalars now implement `to_numpy()` as aliases to `Timestamp.to_datetime64` and `Timedelta.to_timedelta64`, respectively.

- [x] closes #24653
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
