feat: add mellow.nvim theme plugin with lualine support

modules/plugins/statusline/lualine/supported_themes.nix

@@ -5,4 +5,5 @@
   "oxocarbon"
   "gruvbox"
   "nord"
+  "mellow"
 ]

modules/plugins/theme/supported-themes.nix

@@ -303,4 +303,13 @@ in {
 
     styles = ["hard" "medium" "soft"];
   };
+
+  mellow = {
+    setup = {transparent ? false, ...}: ''
+      -- Mellow configuration
+      vim.g.mellow_transparent = ${boolToString transparent}
+
+      vim.cmd.colorscheme("mellow")
+    '';
+  };
 }