Launch another Ruby Interpreter when IRB is started 

from https://github.com/ruby/irb/pull/254#issuecomment-871698900

Separation running and input interpreters sounds good. But I know it's never easy. 

At first I can do is creating issue so I do.

## Expected benefits
- Running something(ex compiling) to check the code on background does not effect the input interpreter
- Reduce `requre`
- maybe something more...