add steel-language-server to nix flake

[cgahr]

I just learned that there is a language server for steel. This is so great!

However, it would be even nicer if this language server would be installable using nix flakes. If this is of interest for this project, I could take care of it.

[mattwparas]

I think that would be a good idea, and would appreciate it if you added that.

I don't use nix - so I don't have much skin in the game there.