add nix infrastructure to build the grammar

shell.nix

@@ -0,0 +1,11 @@
+{ pkgs ? import <nixpkgs> { } }:
+
+let
+  tree-sitter-nixshell = pkgs.callPackage ./. { };
+in
+pkgs.mkShell {
+  inherit (tree-sitter-nixshell) nativeBuildInputs;
+  # to manually build, once in this shell, just run
+  # `tree-sitter generate`
+}
+