make system paths an list that can be managed by shell modules.

[Eveeifyeve]

I am request/suggesting that the enviroment.systemPath should contain an list that contains the paths but the problem is that for shells like nushell setting the path is not via the normal way with nushell it's set via $env.PATH which becomes an issue when I want to make a module for nushell in nix-darwin. There is two ways of solving it, one adding a long if statement for all the sh languages shells (which is not recommended), or two make the shells handle it in the modules.