Proposal: add EnergyTraits

I propose adding a dependency to Unitful.jl based on a new trait package [EnergyTraits.jl](https://github.com/JuliaAstro/EnergyTraits.jl). As a proof of concept, these traits allow functional dispatch on the units involved in the equation `E = h * nu = h * c0 / lambda`.

I have yet to register this package, because I want to see what the feelings over here are. Part of my idea of making EnergyTraits potentially so powerful is having it work with units out of the box. To that end, I have tried to make the package as lightweight as possible (no dependencies, 1 function, 3 types) and I've created a function that automatically tests the trait interface, so actually adding the code should be very easy.

**What would the changes look like?**

Well, I took a stab at this already in this snippet https://github.com/JuliaAstro/EnergyTraits.jl/blob/d21a2c46cb430ebc6c758937a1f469bfa046bb5f/test/runtests.jl#L7-L9

I'm not positive how the dimensions cascade (i.e., while they always simplify?). If so, would the above lines be sufficient for *all* the units? It works in the example in that test file, so I'm eager!

So, would this be a welcome effort in Unitful.jl? I am happy to make the necessary PRs, and add any documentation requested. Thanks for your time!

Start by reviewing the proposed EnergyTraits.jl interface and the referenced test/runtests.jl lines 7-9, then inspect Unitful.jl's existing trait and unit-dispatch APIs. The issue does not name a Unitful.jl target file or test; done would require an agreed integration design and confirmation that the trait behavior works across the relevant units.

Written by the indexing model from the issue text.