shell: Introduce treefmt

Introduces treefmt with a simple nixfmt-rfc-style configuration to
format all Nix files.

This is only practically usable with the following commit that formats
all files accordingly.

shell.nix

@@ -16,7 +16,7 @@
   nixpkgs ? null,
 }:
 let
-  inherit (import ./ci { inherit nixpkgs system; }) pkgs;
+  inherit (import ./ci { inherit nixpkgs system; }) pkgs fmt;
 
   # For `nix-shell -A hello`
   curPkgs = builtins.removeAttrs (import ./. { inherit system; }) [
@@ -28,10 +28,10 @@ let
 in
 curPkgs
 // pkgs.mkShellNoCC {
+  inputsFrom = [
+    fmt.shell
+  ];
   packages = with pkgs; [
-    # The default formatter for Nix code
-    # See https://github.com/NixOS/nixfmt
-    nixfmt-rfc-style
     # Helper to review Nixpkgs PRs
     # See CONTRIBUTING.md
     nixpkgs-review