Add a flake formatter (so nix fmt works)

I have my editor configured to run `nix fmt` on files I edit, so this is
quite helpful for me.

Author: jfly

default.nix

@@ -140,4 +140,6 @@ build
   inherit checks;
 
   ci = pkgs.linkFarm "ci" checks;
+
+  treefmt = treefmtEval.config.build.wrapper;
 }

flake.nix

@@ -22,6 +22,8 @@
         checks = result.checks;
 
         devShells.default = result.shell;
+
+        formatter = result.treefmt;
       }
     );
 }