I would like to propose a new grammar in this PR.
I believe that more parameterizing rules can handle more abstract rules if we can switch between rules and actions that are expanded by conditions in order to make rules common.

Syntax is as follows:
```
%rule defined_rule(X, condition): /* empty */
                                | X { $$ = $1; } %if(condition) /* 1 */
                                | %if(condition) X %endif X { $$ = $1; } /* 2 */
                                ;

%%

r_true        : defined_rule(number, %true)
              ;

r_false       : defined_rule(number, %false)
              ;
```

1. It's like a postfix if in Ruby. If condition is false, it is equivalent to missing this line.
2. If statementIf condition is false, it is equivalent to missing RHS between `%if` and`% endif`.

I believe it will solve the problem mentioned in the article below with the tight coupling with Lexer "to disable certain generation rules under certain conditions" and I would like to propose this feature to solve this problem.
https://yui-knk.hatenablog.com/entry/2023/04/04/190413

We can trace the RHS to [f_args](https://github.com/ruby/ruby/blob/2f916812a9b818b432ee7c299e021ec62d4727fb/parse.y#L5523-L5575) > [args_tail](https://github.com/ruby/ruby/blob/2f916812a9b818b432ee7c299e021ec62d4727fb/parse.y#L5487-L5503) > [args_forward](https://github.com/ruby/ruby/blob/2f916812a9b818b432ee7c299e021ec62d4727fb/parse.y#L5586-L5597), where f_args is the RHS of both the lambda argument (f_larglist) and the method definition argument (f_arglist).
So if we can switch between RHS and actions by passing parameters, we can break up the Lexer/Parser coupling here.

```
%rule defined_rule(X, condition): X { $$ = $1; } %if(condition)
                                ;
```