Add ASCII aliases for Unicode dimension characters 𝕄, 𝕃, 𝕋

I've been wrestling with Windows fonts' broken representations of the mathbb dimensions characters while trying to extend Unitful. It'd be nice to have aliases (e.g. `const MASS, LENGTH, TIME = 𝕄, 𝕃, 𝕋`) for terminal environments where those characters are unrepresentable.

Locate the definitions of the Unicode dimension characters 𝕄, 𝕃, and 𝕋 in the Unitful.jl source and inspect nearby tests or exports. Add ASCII aliases such as MASS, LENGTH, and TIME for terminal environments, then verify that the aliases resolve to the corresponding dimension constants.

Written by the indexing model from the issue text.