[ruby/irb] Refactor exit command

(https://github.com/ruby/irb/pull/835)

* Remove unnecessary code from the exit command's implementation

1. The parameters of `IRB.irb_exit` were never used. But there are some
   libraries seem to call it with arguments + it's declared on the top-level
   IRB constant. So I changed the params to anonymous splat instead of removing them.
2. `Context#exit` was completely unnecessary as `IRB.irb_exit` doesn't use
   the `@irb` instance it passes. And since it's (or should be treated as)
   a private method, I simply removed it.
3. The `exit` command doesn't use the status argument it receives at all.
   But to avoid raising errors on usages like `exit 1`, I changed the argument to
   anonymous splat instead removing it.

* Make exit an actual command

* Update readme

https://github.com/ruby/irb/commit/452b543a65

lib/irb.rb

@@ -886,8 +886,8 @@ module IRB
   end
 
   # Quits irb
-  def IRB.irb_exit(irb, ret)
-    throw :IRB_EXIT, ret
+  def IRB.irb_exit(*)
+    throw :IRB_EXIT
   end
 
   # Aborts then interrupts irb.

lib/irb/cmd/exit.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+require_relative "nop"
+
+module IRB
+  # :stopdoc:
+
+  module ExtendCommand
+    class Exit < Nop
+      category "IRB"
+      description "Exit the current irb session."
+
+      def execute(*)
+        IRB.irb_exit
+      rescue UncaughtThrowError
+        Kernel.exit
+      end
+    end
+  end
+
+  # :startdoc:
+end

lib/irb/context.rb

@@ -573,14 +573,6 @@ module IRB
       @inspect_method.inspect_value(@last_value)
     end
 
-    alias __exit__ exit
-    # Exits the current session, see IRB.irb_exit
-    def exit(ret = 0)
-      IRB.irb_exit(@irb, ret)
-    rescue UncaughtThrowError
-      super
-    end
-
     NOPRINTING_IVARS = ["@last_value"] # :nodoc:
     NO_INSPECTING_IVARS = ["@irb", "@io"] # :nodoc:
     IDNAME_IVARS = ["@prompt_mode"] # :nodoc:

lib/irb/extend-command.rb

@@ -16,15 +16,6 @@ module IRB # :nodoc:
     # See #install_alias_method.
     OVERRIDE_ALL = 0x02
 
-    # Quits the current irb context
-    #
-    # +ret+ is the optional signal or message to send to Context#exit
-    #
-    # Same as <code>IRB.CurrentContext.exit</code>.
-    def irb_exit(ret = 0)
-      irb_context.exit(ret)
-    end
-
     # Displays current configuration.
     #
     # Modifying the configuration is achieved by sending a message to IRB.conf.
@@ -35,13 +26,16 @@ module IRB # :nodoc:
     @ALIASES = [
       [:context, :irb_context, NO_OVERRIDE],
       [:conf, :irb_context, NO_OVERRIDE],
-      [:irb_quit, :irb_exit, OVERRIDE_PRIVATE_ONLY],
-      [:exit, :irb_exit, OVERRIDE_PRIVATE_ONLY],
-      [:quit, :irb_exit, OVERRIDE_PRIVATE_ONLY],
     ]
 
 
     @EXTEND_COMMANDS = [
+      [
+        :irb_exit, :Exit, "cmd/exit",
+        [:exit, OVERRIDE_PRIVATE_ONLY],
+        [:quit, OVERRIDE_PRIVATE_ONLY],
+        [:irb_quit, OVERRIDE_PRIVATE_ONLY],
+      ],
       [
         :irb_current_working_workspace, :CurrentWorkingWorkspace, "cmd/chws",
         [:cwws, NO_OVERRIDE],