Widen the definable position of parameterizing rules

We came to this decision after discussion at RubyKaigi 2024.
Definitions are also possible in the following positions:

```
%rule defined_option(X): /* empty */
                       | X
                       ;

%%

program         : defined_option(number) <i>
                | defined_list(number) <i>
                ;

%rule defined_list(X): /* empty */  /* <--- here */
                     | defined_list(X) number
                     ;
```