Add support for modular services

[Eveeifyeve]

Related discussion: https://discourse.nixos.org/t/modular-services-meeting-3-2026-06-26/78582#p-263271-sync-2
Docs: https://nixos.org/manual/nixos/unstable/#modular-services

It could be something like: config.services.<name>

[Eveeifyeve]

It would be good to look into a function that implements _class = "service" automatically and importApply before we add support for this.