Parse function definitions more strictly

grammar.js

@@ -123,9 +123,10 @@ module.exports = grammar({
     ),
 
     function_definition: $ => seq(
-      optional('function'),
+      choice(
-      $.word,
+        seq('function', $.word, optional(seq('(', ')'))),
-      optional(seq('(', ')')),
+        seq($.word, '(', ')')
+      ),
       $.compound_statement,
       optional($.file_redirect)
     ),