fix: Add missing treefmt.nix

Fixes commit 0ef0d4652a, file was missing and tests failed
https://builds.auxolotl.org/#/builders/35/builds/1

treefmt.nix

@@ -0,0 +1,15 @@
+_: {
+  # Used to find the project root
+  projectRootFile = "flake.nix";
+
+  # Files not to format.
+  settings.global.excludes = [ "LICENSE" ];
+
+  # nix
+  programs.nixfmt.enable = true; # As of 2024-07-30 this is nixfmt-rfc-style.
+  programs.statix.enable = true;
+  programs.deadnix.enable = true;
+
+  # markdown
+  programs.prettier.enable = true;
+}