add 'as' as arcsecond for alias with prefixes enabled

[mileslucas]

It is extremely common in astronomy to use arcseconds. Very commonly, we work with quantities like "milliarcseconds" and "microarcseconds" for the super small angles on the night sky. We've run into this issue in (JuliaAstro/UnitfulAstro.jl#7). This PR adds a new unit "as" (ArcsecondShort struct name) which has prefixes enabled. The following conversions are now trivial-

julia> using Unitful, UnitfulAngles

julia> plate_scale = 0.91u"mas/mm"
0.91 mas mm⁻¹

julia> plate_scale |> u"rad/m"
4.411804498096778e-6 rad m⁻¹

julia> ustrip(ans) ≈ deg2rad(ustrip(plate_scale) * 1e3 / 3.6e6)
true

I've also added as to the testing units, and have added extra tests to ensure the prefixes are accurate alongside what Unitful provides.

Current Problems

There are two problems to address that I would like feedback on, if possible

• u"as" is technically valid as "attoseconds". I don't know a good workaround for this- if there was a way to disable u"as" while still allowing, e.g., u"mas" that would be perfect, since it's easy enough to write out u"arcsecond" if the prefix isn't needed.
• I can't find any clear examples of essentially aliasing a unit in Unitful.jl, although we are aliasing Arcsecond with this new ArcsecondShort unit. I don't know if this is the appropriate way and I don't love it, since it seems very easy to get mixed up.

One of the alternatives would be adding two bespoke units for millarcsecond and microarcsecond, but that feels very ad-hoc and adds more potential for bugs between the three Unitful libraries I've mentioned

[yakir12]

Thanks a lot for this. I don't have a lot of time to invest in this package (also see this comment), but I trust your judgement, so I'll merge this u"as" is (heh).