LibJS: Disallow standalone super expression

Author: https://github.com/davidot
Commit: 4d6502de42
Pull-request: https://github.com/SerenityOS/serenity/pull/9085
Reviewed-by: https://github.com/IdanHo
Reviewed-by: https://github.com/linusg ✅
Reviewed-by: https://github.com/trflynn89

Userland/Libraries/LibJS/Parser.cpp

@@ -1297,6 +1297,9 @@ NonnullRefPtr<Expression> Parser::parse_expression(int min_precedence, Associati
         }
     }
 
+    if (is<SuperExpression>(*expression))
+        syntax_error("'super' keyword unexpected here");
+
     check_for_invalid_object_property(expression);
 
     if (match(TokenType::Comma) && min_precedence <= 1) {