consider adding month as a unit of time

Hello,

Please consider adding the month as a unit of time into pkgdefaults.jl, like so

" Unitful.mo
\nThe month, a unit of time, defined as 1/12yr.
\nDimension: [`Unitful.𝐓`](@ref).
\nSee Also: [`Unitful.yr`](@ref)."
@unit mo "mo" Month 1/12yr false

This is a widely used unit in atmospheric and oceanic sciences.

Also thanks for an amazing package.

Start in pkgdefaults.jl and compare the proposed month definition with nearby time-unit declarations. Add the documented mo unit with the stated 1/12yr definition; the work is done when the month unit is available with the requested documentation.

Written by the indexing model from the issue text.