config: add treefmt as formatter tool

flake.nix

@@ -388,7 +388,7 @@
       # No amount of formatters will make this codebase nicer but it sure does
       # feel like it does.
       formatter =
-        forAllSystems (system: nixpkgs.legacyPackages.${system}.nixpkgs-fmt);
+        forAllSystems (system: nixpkgs.legacyPackages.${system}.treefmt);
 
       # nixops-lite... in here!?! We got it all, son!
       #

shell.nix

@@ -8,10 +8,16 @@ pkgs.mkShell {
     jq
     nixpkgs-fmt
     sops
+    treefmt
 
     # Language servers for various parts of the config that uses a language.
     sumneko-lua-language-server
     pyright
     rnix-lsp
+
+    # Formatters...
+    stylua # ...for Lua.
+    black # ...for Python.
+    nixpkgs-fmt # ...for Nix.
   ];
 }

treefmt.toml

@@ -0,0 +1,11 @@
+[formatter.lua]
+command = "stylua"
+includes = [ "*.lua" ]
+
+[formatter.nix]
+command = "nixpkgs-fmt"
+includes = [ "*.nix" ]
+
+[formatter.python]
+command = "black"
+includes = [ "*.py" ]