Declaratively enable user service in NixOS module

[Majiir]

As I recently learned, NixOS supports enabling user services for all users declaratively. It would be great to add this to the NixOS module for nixos-vscode-server to make usage easier and eliminate the manual symlinking steps.

[Flameopathic]

i second this. it's not a hard process, and this system is not currently declarative.

[Ten0]

I have worked out some code that does this somewhere... I should PR it into this module.