fix a regression (GDScript) from e00630b

This removes `not` from the variable safe list of
keywords.
Before that this was a valid expression:
    self.!(some_arg)

The other fix is just a forgotten boolean negation.

modules/gdscript/gd_parser.cpp

@@ -3283,7 +3283,7 @@ void GDParser::_parse_class(ClassNode *p_class) {
 							break;
 						}
 
-						if (tokenizer->is_token_literal(0, true)) {
+						if (!tokenizer->is_token_literal(0, true)) {
 							_set_error("Expected identifier in signal argument.");
 							return;
 						}