Deprecate multi-irb feature

Plan

• Print deprecation warnings on these commands: irb, jobs, fg, and kill in the next minor release
• Completely remove those commands in v2.0.0

Reasons

• It's concept highly overlaps with workspace commands like chws, pushws...etc.
• It spawns a new thread for every new new session, which conflicts with the upcoming irb-debug integration
• But most importantly, we can't find any practical use for this feature and the cost of maintaining its compatibility with other features is high
  • For example, it will require the solution of #650 to support multi-thread irb sessions too

Feedback wanted

If you use the multi-irb feature and don't want to see it deprecated from IRB, please leave a comment below to explain your use case of it. It will help us assess this plan.

Beginne damit, die Implementierungen und die Befehlsverarbeitung für irb, jobs, fg und kill zu finden, und untersuche anschließend, wie IRB-Versionen und Deprecations verwaltet werden. Als abgeschlossen gilt die Aufgabe, wenn diese Befehle im nächsten Minor-Release Warnungen ausgeben und in v2.0.0 entfernt werden und die relevanten Tests aktualisiert sind.