Update to (somewhat?) dendritic pattern

Heavily inspired by https://github.com/Doc-Steve/dendritic-design-with-flake-parts.

> Atomic commits? Never heard of them!

modules/programs/nixvim/nixvim.nix

@@ -0,0 +1,19 @@
+{ inputs, ... }: {
+  flake-file.inputs = {
+    nixvim.url = "github:nix-community/nixvim/nixos-25.11";
+    nixvim.inputs.nixpkgs.follows = "nixpkgs";
+  };
+
+  flake.modules.homeManager.nixvim = {
+    imports = [
+      inputs.nixvim.homeModules.nixvim
+    ];
+
+    programs.nixvim = {
+      enable = true;
+      imports = [ ./_nixvim-config ];
+    };
+
+    home.sessionVariables.EDITOR = "nvim";
+  };
+}