Replace nodePackages with top-level equivalents

[lumi-a]

With 26.05, nodePackages has been removed:
https://nixos.org/manual/nixpkgs/stable/release-notes#sec-nixpkgs-release-26.05-incompatibilities

This simply updates all the nodePackages to their top-level equivalents:

• nodejs
• pnpm
• typescript
• typescript-language-server
• markdown-link-check

For example, before this commit, using the pnpm template yielded:

error:
  … while calling the 'derivationStrict' builtin
    at «nix-internal»/derivation-internal.nix:37:12:
      36|
      37|   strict = derivationStrict drvAttrs;
        |            ^
      38|

  … while evaluating derivation 'nix-shell'
    whose name attribute is located at «github:NixOS/nixpkgs/d407951447dcd00442e97087bf374aad70c04cea?narHash=sha256-8i/87eeoqiGE4yOTjwSA3Eh/ziJRQEmd/unYU%2BK27sk%3D»/pkgs/stdenv/generic/make-derivation.nix:651:11

  … while evaluating attribute 'buildInputs' of derivation 'nix-shell'
    at «github:NixOS/nixpkgs/d407951447dcd00442e97087bf374aad70c04cea?narHash=sha256-8i/87eeoqiGE4yOTjwSA3Eh/ziJRQEmd/unYU%2BK27sk%3D»/pkgs/stdenv/generic/make-derivation.nix:722:11:
     721|           depsHostHost = hostHostOutputs;
     722|           buildInputs = hostTargetOutputs;
        |           ^
     723|           depsTargetTarget = targetTargetOutputs;

  (stack trace truncated; use '--show-trace' to show the full, detailed trace)

  error: nodePackages has been removed. Many packages are now available at the top level (e.g. `pkgs.package-name`). Check on https://search.nixos.org to see if the package is still available.