Allow executing debugging commands from irb session

[st0012]

Description

Currently, irb supports 10 debugging commands that are powered by debug, which is a huge first-step improvement.

However, when users run those commands, they'll enter a new debug session and leave irb. This could be inconvenient because:

• debug doesn't support multi-line mode
• debug doesn't have autocompletion yet
• irb and debug has different commands or behaviour
• Users will lose local variables defined in the current irb session

So it'd be amazing if we can let users run the same debugging commands, while letting them stay inside the current irb session.