refactor: move modules to their own dir

modules/nixvim/default.nix

@@ -0,0 +1,17 @@
+{ lib, config, ... }:
+let
+  cfg = config.jhome.nvim;
+in
+{
+  imports = [ ./options.nix ];
+
+  config.programs.nixvim = lib.mkMerge [
+    (import ./standalone.nix)
+    (lib.mkIf cfg.enable {
+      enable = true;
+      nixpkgs.useGlobalPackages = true;
+      defaultEditor = lib.mkDefault true;
+      jhome.nvim = cfg;
+    })
+  ];
+}