modules/nixos: add systemd services

This adds systemd services configuration through hjem, under
`users.<username>.systemd.services`.

modules/nixos/systemd/services.nix

@@ -0,0 +1,57 @@
+{
+  lib,
+  config,
+  ...
+}: let
+  inherit (builtins) toString;
+  inherit (lib.attrsets) nameValuePair mapAttrs';
+  inherit (lib.modules) mkIf;
+  inherit (lib.options) mkOption;
+  inherit (lib.types) attrs attrsOf submodule;
+  inherit (lib.trivial) isBool boolToString;
+  inherit (lib.generators) toINI;
+
+  cfg = config.systemd;
+
+  toSystemdUnitFiles = services: let
+    toSystemdUnit = arg:
+      toINI {
+        listsAsDuplicateKeys = true;
+        mkKeyValue = key: value: let
+          value' =
+            if isBool value
+            then boolToString value
+            else toString value;
+        in "${key}=${value'}";
+      }
+      arg;
+  in
+    mapAttrs' (name: service:
+      nameValuePair ".config/systemd/user/${name}.service" {text = toSystemdUnit service.settings;})
+    services;
+in {
+  options.systemd = {
+    services = mkOption {
+      type = attrsOf (submodule {
+        options = {
+          settings = mkOption {
+            type = attrs;
+            default = {};
+            description = ''
+              The configuration of this unit. Each attribute in this set specifies an option
+              (documentation for available options can be found [here](https://www.freedesktop.org/software/systemd/man/latest/systemd.service.html)).
+            '';
+          };
+        };
+      });
+      default = {};
+      description = ''
+        Definition of systemd user service units.
+      '';
+    };
+  };
+
+  config = mkIf (cfg.services != {}) {
+    files = toSystemdUnitFiles cfg.services;
+  };
+}