ENH: to_offset "year"/"quarter"/"month"/"day"/...?
In the process of untangling Resolution/FreqGroup/PeriodDtype/Offsets im finding it would be helpful if `to_offset("year")` behaved like `to_offset("Y")` etc.  But apparently these are specifically [deprecated](https://github.com/pandas-dev/pandas/blob/master/pandas/tests/scalar/period/test_period.py#L724).  

This test was introduced [here](https://github.com/pandas-dev/pandas/commit/4da9b15f58cfcde998d6279a90d55a170145f8c2) (commit but no PR) and is listed as closing #11854 and #12540, but neither of those really make it clear to me why we wouldn't allow these.

Any objection to having to_offset recognize these strings?