Deprecate multi-irb commands

- Print deprecated message when any of the commands are used
- Put related commands under `Multi-irb` category with a deprecated
  label

lib/irb/cmd/subirb.rb

@@ -18,45 +18,56 @@ def initialize(conf)
 
       private
 
+      def print_deprecated_warning
+        warn <<~MSG
+          Multi-irb commands are deprecated and will be removed in IRB 2.0.0. Please use workspace commands instead.
+          If you have any use case for multi-irb, please leave a comment at https://github.com/ruby/irb/issues/653
+        MSG
+      end
+
       def extend_irb_context
         # this extension patches IRB context like IRB.CurrentContext
         require_relative "../ext/multi-irb"
       end
     end
 
     class IrbCommand < MultiIRBCommand
-      category "IRB"
+      category "Multi-irb (DEPRECATED)"
       description "Start a child IRB."
 
       def execute(*obj)
+        print_deprecated_warning
         IRB.irb(nil, *obj)
       end
     end
 
     class Jobs < MultiIRBCommand
-      category "IRB"
+      category "Multi-irb (DEPRECATED)"
       description "List of current sessions."
 
       def execute
+        print_deprecated_warning
         IRB.JobManager
       end
     end
 
     class Foreground < MultiIRBCommand
-      category "IRB"
+      category "Multi-irb (DEPRECATED)"
       description "Switches to the session of the given number."
 
       def execute(key = nil)
+        print_deprecated_warning
         raise CommandArgumentError.new("Please specify the id of target IRB job (listed in the `jobs` command).") unless key
         IRB.JobManager.switch(key)
       end
     end
 
     class Kill < MultiIRBCommand
-      category "IRB"
+      category "Multi-irb (DEPRECATED)"
       description "Kills the session with the given number."
 
       def execute(*keys)
+        print_deprecated_warning
         IRB.JobManager.kill(*keys)
       end
     end