ENH: is_scalar returns True for DateOffset objects
- [x] closes #18943
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

I've implemented `is_offset` identically to `is_period_object` and added it to `isscalar` and `is_scalar` is now returning true for DateOffsets. 

But not sure how to go about this exactly:
> `is_offset` should also be imported / tested in `pandas/core/dtypes/common.py`

The other "is" functions that are explicitly imported from inference `is_string_like` and `is_list_like` are used for testing but don't look like they themselves are being tested so I'm not sure what kind of test is needed. The rest of the "is" functions are imported with a wildcard but pylint is telling me they are not used (should I go through and make the required imports explicit?).




