nixos: create 'modules/nixos' folder

Let's consolidate all modules under one path, so that NixOS,
home-manager, and nix-darwin (if I ever end up using it down the line)
would go under the same folder.

modules/nixos/hardware/upower/default.nix

@@ -0,0 +1,44 @@
+{ config, lib, ... }:
+let
+  cfg = config.my.hardware.upower;
+in
+{
+  options.my.hardware.upower = with lib; {
+    enable = mkEnableOption "upower configuration";
+
+    levels = {
+      low = mkOption {
+        type = types.ints.unsigned;
+        default = 25;
+        example = 10;
+        description = "Low percentage";
+      };
+
+      critical = mkOption {
+        type = types.ints.unsigned;
+        default = 15;
+        example = 5;
+        description = "Critical percentage";
+      };
+
+      action = mkOption {
+        type = types.ints.unsigned;
+        default = 5;
+        example = 3;
+        description = "Percentage at which point an action must be taken";
+      };
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    services.upower = {
+      enable = true;
+
+      percentageLow = cfg.levels.low;
+
+      percentageCritical = cfg.levels.critical;
+
+      percentageAction = cfg.levels.action;
+    };
+  };
+}