replace vscode with helix

vscode is just too slow on my notebook, use helix with marksman as
alternative

flake.nix

@@ -10,7 +10,7 @@
       in {
         # For `nix develop`:
         devShell = pkgs.mkShell {
-          nativeBuildInputs = with pkgs; [ vscodium ];
+          nativeBuildInputs = with pkgs; [ helix marksman lazygit ];
         };
       });
 }