treefmt: add keep-sorted

Signed-off-by: Paul Meyer <[email protected]>

treefmt.nix

@@ -2,12 +2,15 @@ _:
 {
   projectRootFile = "flake.nix";
   programs = {
-    nixpkgs-fmt.enable = true;
+    # keep-sorted start
     deadnix.enable = true;
+    just.enable = true;
+    keep-sorted.enable = true;
+    nixpkgs-fmt.enable = true;
     shellcheck.enable = true;
     shfmt.enable = true;
     statix.enable = true;
     yamlfmt.enable = true;
-    just.enable = true;
+    # keep-sorted end
   };
 }