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.

まず irb、jobs、fg、kill の実装とコマンド処理を見つけ、次に IRB のバージョンと非推奨化がどのように管理されているかを調べます。これらのコマンドが次のマイナーリリースで警告を出し、v2.0.0 で削除され、関連するテストが更新されていれば完了です。