feat(nix): add hjem-module based on home-manager module

[Nerglej]

Hi there!

I'm migrating my config to hjem from home-manager. Since they're very much alike module-wise, I made a common config helper that configures both the home-manager and hjem modules. I get identical output with home-manager before and after my changes, and also with hjem now.

I made some decisions that I would like opinions on:

• I chose to call the hjem option desktops.mango, since there's no established hjem system except for how hjem-rum does it.
• Added two outputs, both mango.hjemModules.mango and mango.hjemModules.default, since I've seen mango.hjemModules.default more often in the hjem ecosystem, e.g. hjem-rum and noctalia
• I decided to let the warning about the legacy text based config stay for home-manager only, since this hjem-module has never had the text based config.

Have a nice day :D

[Nerglej]

I just realized that I formatted flake.nix. Do we have any nix styling guidelines?

[rysieko]

for formatter there's outputed formatter = pkgs.alejandra; in the flake, set your LSP to format using nix fmt