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.

UI/default.nix

@@ -1,20 +0,0 @@
-{ pkgs ? import <nixpkgs> { } }:
-
-pkgs.mkShell {
-  packages = with pkgs; [
-    ccache
-  ] ++ (with qt6Packages; [
-    qtbase.dev
-    qttools
-    qtwayland.dev
-  ]);
-
-  inputsFrom = [ pkgs.ladybird ];
-
-  shellHook = ''
-    # NOTE: This is required to make it find the wayland platform plugin installed
-    #       above, but should probably be fixed upstream.
-    export QT_PLUGIN_PATH="$QT_PLUGIN_PATH:${pkgs.qt6.qtwayland}/lib/qt-6/plugins"
-    export QT_QPA_PLATFORM="wayland;xcb"
-  '';
-}