project: add codebase formatter

flake.nix

@@ -8,8 +8,13 @@
 
   outputs = inputs@{ self, nixpkgs, ... }:
     let systems = inputs.flake-utils.lib.defaultSystems;
-    in inputs.flake-utils.lib.eachSystem systems (system: {
+    in inputs.flake-utils.lib.eachSystem systems (system: let
+      pkgs = import nixpkgs { inherit system; };
+    in
+    {
       devShell =
-        import ./shell.nix { pkgs = import nixpkgs { inherit system; }; };
+        import ./shell.nix { inherit pkgs; };
+
+      formatter = pkgs.treefmt;
     });
 }

shell.nix

@@ -16,7 +16,17 @@ with pkgs;
 mkShell {
   packages = [
     python3
-    pyright
     stow
+
+    # Language servers for...
+    lua-language-server # ...Lua.
+    pyright # ...Python.
+    rnix-lsp # ...Nix.
+
+    # Formatters for...
+    treefmt # ...everything under the sun.
+    stylua # ...Lua.
+    nixpkgs-fmt # ...Nix.
+    black # ...Python.
   ];
 }

treefmt.toml

@@ -0,0 +1,3 @@
+[formatter.lua]
+command = "stylua"
+includes = [ "*.lua" ]