refactor(modules/nixos): move stuff around

This makes the module a bit clearer and easier to extend.

modules/lib.nix

@@ -0,0 +1,27 @@
+{ lib }:
+let
+  inherit (lib) types;
+in
+{
+  # Like mkEnableOption but defaults to true
+  mkDisableOption =
+    option:
+    (lib.mkEnableOption option)
+    // {
+      default = true;
+      example = false;
+    };
+  # A option that accepts an image (and shows it in the docs)
+  mkImageOption =
+    {
+      description,
+      url,
+      sha256 ? "",
+    }:
+    lib.mkOption {
+      inherit description;
+      type = types.path;
+      default = builtins.fetchurl { inherit url sha256; };
+      defaultText = lib.literalMD "![${description}](${url})";
+    };
+}