Add a `SlackNLPModelMeta`

[tmigot]

I added these entries to the model directly for now.
Adding, the new meta will be approx. 100 lines of codes https://github.com/JuliaSmoothOptimizers/NLPModels.jl/blob/main/src/nlp/meta.jl
So, I suggest I do it in a separate PR.

Originally posted by @tmigot in #48 (comment)