Flake: Add and run formatter + Rename devshell

Add a formatter output to the flake (`nix fmt`), along with moving +
renaming the devshell so it will work by running `nix-shell` in the root
of the project.

Documentation/BuildInstructionsLadybird.md

@@ -114,10 +114,10 @@ nix develop
 nix develop --command bash
 
 # Using nix-shell
-nix-shell UI
+nix-shell
 
 # Using nix-shell and a custom shell
-nix-shell UI --command bash
+nix-shell --command bash
 ```
 
 Then invoke `cmake` directly. For example:

flake.nix

@@ -3,12 +3,24 @@
 
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
-    utils.url = "github:numtide/flake-utils";
+    flake-utils.url = "github:numtide/flake-utils";
   };
 
-  outputs = { self, nixpkgs, utils, }: utils.lib.eachDefaultSystem (system: let
-    pkgs = import nixpkgs { inherit system; };
-  in {
-    devShells.default = import ./UI { inherit pkgs; };
-  });
+  outputs =
+    {
+      nixpkgs,
+      flake-utils,
+      ...
+    }:
+    flake-utils.lib.eachDefaultSystem (
+      system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+      in
+      {
+        devShells.default = import ./shell.nix { inherit pkgs; };
+
+        formatter = pkgs.nixfmt-rfc-style;
+      }
+    );
 }

UI/default.nix → shell.nix

@@ -1,15 +1,19 @@
-{ pkgs ? import <nixpkgs> { } }:
+{
+  pkgs ? import <nixpkgs> { },
+}:
 
 pkgs.mkShell {
-  packages = with pkgs; [
-    ccache
-  ] ++ (with qt6Packages; [
+  inputsFrom = [
+    pkgs.ladybird
+  ];
+
+  packages = with pkgs.qt6Packages; [
     qtbase.dev
     qttools
     qtwayland.dev
-  ]);
 
-  inputsFrom = [ pkgs.ladybird ];
+    pkgs.ccache
+  ];
 
   shellHook = ''
     # NOTE: This is required to make it find the wayland platform plugin installed