refactoring to more granular flakes and modules

flakes/common/nix_modules/essentials/nixpkg.func.sh

@@ -0,0 +1,13 @@
+# nix
+alias nixpkgs=nixpkg
+nixpkg () {
+  if [ $# -eq 0 ]; then
+    echo "Error: No arguments provided. Please specify at least one package."
+    return 1
+  fi
+  cmd="nix shell"
+  for pkg in "$@"; do
+    cmd="$cmd \"nixpkgs#$pkg\""
+  done
+  eval $cmd
+}