ghostty: allow darwin users to manager their config

Currently nixpkgs does not contain a package defintion for ghostty
compatible with Darwin. Darwin users may still want to use this module
to manage their config or share config between systems. This carries
over behaviour from the beta period where this same technique was used.

see: https://github.com/clo4/ghostty-hm-module/blob/887e13a6e7acf5ffaab0119d96e476d84db90904/module.nix#L167-L173

Also improves validation to cover theme files.

modules/programs/ghostty.nix

@@ -13,7 +13,11 @@ in {
   options.programs.ghostty = {
     enable = lib.mkEnableOption "Ghostty";
 
-    package = lib.mkPackageOption pkgs "ghostty" { };
+    package = lib.mkPackageOption pkgs "ghostty" {
+      nullable = true;
+      extraDescription =
+        "On darwin set this to null to manage ghostty config while ghostty is not present in nixpkgs.";
+    };
 
     settings = lib.mkOption {
       inherit (keyValue) type;
@@ -78,7 +82,7 @@ in {
     installBatSyntax =
       lib.mkEnableOption "installation of Ghostty configuration syntax for bat"
       // {
-        default = true;
+        default = (cfg.package != null);
       };
 
     enableBashIntegration = lib.mkEnableOption ''
@@ -108,35 +112,53 @@ in {
 
   config = lib.mkIf cfg.enable (lib.mkMerge [
     {
-      home.packages = [ cfg.package ];
+      home.packages = lib.mkIf (cfg.package != null) [ cfg.package ];
 
       programs.ghostty.settings = lib.mkIf cfg.clearDefaultKeybinds {
         keybind = lib.mkBefore [ "clear" ];
       };
 
       # MacOS also supports XDG configuration directory, so we use it for both
       # Linux and macOS to reduce complexity
-      xdg.configFile = lib.mkMerge [
+      xdg.configFile = let
+        validate = file:
+          lib.mkIf (cfg.package != null) "${
+            lib.getExe cfg.package
+          } +validate-config --config-file=${config.xdg.configHome}/ghostty/${file}";
+      in lib.mkMerge [
         {
           "ghostty/config" = lib.mkIf (cfg.settings != { }) {
             source = keyValue.generate "ghostty-config" cfg.settings;
-            onChange = "${lib.getExe cfg.package} +validate-config";
+            onChange = validate "config";
           };
         }
 
         (lib.mkIf (cfg.themes != { }) (lib.mapAttrs' (name: value: {
           name = "ghostty/themes/${name}";
-          value.source = keyValue.generate "ghostty-${name}-theme" value;
+          value = {
+            source = keyValue.generate "ghostty-${name}-theme" value;
+            onChange = validate "themes/${name}";
+          };
         }) cfg.themes))
       ];
     }
 
     (lib.mkIf cfg.installVimSyntax {
-      programs.vim.plugins = [ cfg.package.vim ];
+      assertions = [{
+        assertion = lib.elem "vim" cfg.package.outputs or [ ];
+        message = "ghostty.package does not have a vim output or is null";
+      }];
+
+      programs.vim.plugins = lib.mkIf (cfg.package != null) [ cfg.package.vim ];
     })
 
     (lib.mkIf cfg.installBatSyntax {
-      programs.bat = {
+      assertions = [{
+        assertion = cfg.package != null;
+        message = "cannot install bat syntax while .package is null";
+      }];
+
+      programs.bat = lib.mkIf (cfg.package != null) {
         syntaxes.ghostty = {
           src = cfg.package;
           file = "share/bat/syntaxes/ghostty.sublime-syntax";