more shell requirements for dotfiles

shell.nix

@@ -1,5 +1,10 @@
 # Environment with formatting tools for editing these files
 { pkgs ? import <nixpkgs> { } }:
 pkgs.mkShell {
-  nativeBuildInputs = [ pkgs.buildPackages.stylua pkgs.buildPackages.nixfmt ];
+  nativeBuildInputs = [
+    pkgs.buildPackages.stylua
+    pkgs.buildPackages.nixfmt
+    pkgs.buildPackages.shfmt
+    pkgs.buildPackages.shellcheck
+  ];
 }