feat(git): replace delta with difftastic

modules/programs/git.nix

@@ -1,5 +1,6 @@
 { config
 , lib
+, pkgs
 , ...
 }:
 
@@ -51,8 +52,8 @@ in
         extraConfig = {
           init.defaultBranch = "main";
           merge.conflictstyle = "zdiff3";
+          diff.external = getExe pkgs.difftastic;
         };
-        delta.enable = true;
         signing = mkIf cfg.signing {
           key = "0x970C6E89EB0547A9";
           signByDefault = true;