feat(ensure): create ensure module

This module serves to ensure specific directories exist, however in the
future it could be used for other things that need to be 'ensured'.

modules/functionality/ensure.nix

@@ -0,0 +1,21 @@
+{ config, lib, pkgs, ... }:
+with lib;
+with lib.hlissner;
+let
+  cfg = config.services.ensure;
+in
+{
+  options.services.ensure = with types; {
+    enable = mkBoolOpt true;
+    directories = mkOpt (listOf string) [ ];
+  };
+
+  config.systemd.services = {
+    "ensure-directories" = mkIf (cfg.enable && (length cfg.directories > 0)) {
+      description = "Ensures certain directories exist (${concatStringsSep "," cfg.directories})";
+      wantedBy = [ "multi-user.target" ];
+      serviceConfig.Type = "oneshot";
+      script = "mkdir -p ${concatStringsSep " " cfg.directories}";
+    };
+  };
+}