leave myself TODOs for the remaining edge cases

grammar.js

@@ -2,6 +2,7 @@ module.exports = grammar({
   name: 'nixshell',
 
   rules: {
+    // TODO: allow trailing whitespace on any line
     source_file: $ => seq($.first_line, repeat($.next_line)),
 
     first_line: $ => seq($._shebang_open, $._opt_ws, optional(seq($._env, $._ws)), $._nix_shell),
@@ -47,6 +48,7 @@ module.exports = grammar({
     ),
 
     _shebang_open: $ => '#!',
+    // TODO: env accepts flags like `-v` or `--unset=NAME` or `VAR=VALUE` before `nix-shell`,
     _env: $ => choice(
       '/usr/bin/env',
       '/bin/env',