It’s this package: https://github.com/nixos/nixpkgs/blob/cd6d4800edca7c9765f40a4416569911d7952b30/pkgs/applications/editors/emacs/elisp-packages/manual-packages/treesit-grammars/default.nix
emacsPackages
isn’t actually part of the name. An example usage is here:
https://github.com/nix-community/emacs-overlay/issues/341#issuecomment-1605290875
You will likely need to change it to look something like
let
myEmacs = (emacsPackagesFor emacs).emacsWithPackages (epkgs: with epkgs; [
vterm
treesit-grammars.with-all-grammars
];
if you’re not using the emacs-overlay.