nixvim: expose config as read-only option

Allow standalone nixvim users to take advantage of stylix by exposing
the generated config as `config.stylix.targets.nixvim.config`.

This can be passed to the nixvim derivation's `extendNixvim` function.

modules/nixvim/nixvim.nix

@@ -11,48 +11,75 @@
       main = lib.mkEnableOption "background transparency for the main NeoVim window";
       sign_column = lib.mkEnableOption "background transparency for the NeoVim sign column";
     };
+    config = lib.mkOption {
+      type = with lib.types; attrsOf anything;
+      readOnly = true;
+
+      description = ''
+        The stylix configuration, generated for nixvim.
+
+        If nixvim is installed via nixos, darwin, or home-manager then this will be **automatically**
+        assigned to `programs.nixvim`. If you're using a "standalone" build of nixvim, then that's
+        not possible. Instead, you should pass this config to the `nixvimExtend` function.
+
+        For example:
+
+        ```nix
+          { config, ... }: {
+            environment.systemPackages = [
+              (standalone-nixvim-derivation.nixvimExtend config.stylix.targets.nixvim.config)
+            ];
+          }
+        ```
+
+        See nixvim's docs on [extending a standalone configuration](https://nix-community.github.io/nixvim/modules/standalone.html#extending-an-existing-configuration).
+      '';
+    };
   };
 
-  config = lib.mkIf ((config.programs ? nixvim) && config.stylix.targets.nixvim.enable) (
-    lib.optionalAttrs (builtins.hasAttr "nixvim" options.programs) {
-      programs.nixvim = {
-        colorschemes.base16 = {
-          colorscheme = let
-            colors = config.lib.stylix.colors.withHashtag;
-          in {
-            base00 = colors.base00;
-            base01 = colors.base01;
-            base02 = colors.base02;
-            base03 = colors.base03;
-            base04 = colors.base04;
-            base05 = colors.base05;
-            base06 = colors.base06;
-            base07 = colors.base07;
-            base08 = colors.base08;
-            base09 = colors.base09;
-            base0A = colors.base0A;
-            base0B = colors.base0B;
-            base0C = colors.base0C;
-            base0D = colors.base0D;
-            base0E = colors.base0E;
-            base0F = colors.base0F;
-          };
-
-          enable = true;
-        };
+  config = {
+    programs = lib.mkIf ((config.programs ? nixvim) && config.stylix.targets.nixvim.enable) (
+      lib.optionalAttrs (builtins.hasAttr "nixvim" options.programs) {
+        nixvim = config.stylix.targets.nixvim.config;
+      }
+    );
+    stylix.targets.nixvim.config = {
+      colorschemes.base16 = {
+        enable = true;
 
-        highlight = let
-          cfg = config.stylix.targets.nixvim;
-          transparent = {
-            bg = "none";
-            ctermbg = "none";
-          };
+        colorscheme = let
+          colors = config.lib.stylix.colors.withHashtag;
         in {
-          Normal = lib.mkIf cfg.transparent_bg.main transparent;
-          NonText = lib.mkIf cfg.transparent_bg.main transparent;
-          SignColumn = lib.mkIf cfg.transparent_bg.sign_column transparent;
+          base00 = colors.base00;
+          base01 = colors.base01;
+          base02 = colors.base02;
+          base03 = colors.base03;
+          base04 = colors.base04;
+          base05 = colors.base05;
+          base06 = colors.base06;
+          base07 = colors.base07;
+          base08 = colors.base08;
+          base09 = colors.base09;
+          base0A = colors.base0A;
+          base0B = colors.base0B;
+          base0C = colors.base0C;
+          base0D = colors.base0D;
+          base0E = colors.base0E;
+          base0F = colors.base0F;
+        };
+      };
+
+      highlight = let
+        cfg = config.stylix.targets.nixvim;
+        transparent = {
+          bg = "none";
+          ctermbg = "none";
         };
+      in {
+        Normal = lib.mkIf cfg.transparent_bg.main transparent;
+        NonText = lib.mkIf cfg.transparent_bg.main transparent;
+        SignColumn = lib.mkIf cfg.transparent_bg.sign_column transparent;
       };
-    }
-  );
+    };
+  };
 }