Add treefmt pre-commit hook

This lets us use different formatters for different filetypes.

Change-Id: Ib52383dd5097c8919a65e299aca2b5a55412223c

flake.nix

@@ -331,6 +331,10 @@
               stages = [ "commit" ];
               excludes = [ "^tests/functional/lang/" ];
             };
+            treefmt = {
+              enable = true;
+              settings.formatters = [ ];
+            };
           };
         }) pre-commit-hooks.lib;
       };

treefmt.toml

@@ -0,0 +1 @@
+[formatter]