Move Session#exnted_feature into Session class

Currently, `exnted_feature` method is defined outside of Session class. This PR fixes it.

Author: ono-max

lib/debug/session.rb

@@ -1993,6 +1993,13 @@ def before_fork need_lock = true
     def after_fork_parent
       @ui.after_fork_parent
     end
+
+    # experimental API
+    def extend_feature session: nil, thread_client: nil, ui: nil
+      Session.include session if session
+      ThreadClient.include thread_client if thread_client
+      @ui.extend ui if ui
+    end
   end
 
   class ProcessGroup
@@ -2147,13 +2154,6 @@ def flush
     end
   end
 
-  # experimental API
-  def extend_feature session: nil, thread_client: nil, ui: nil
-    Session.include session if session
-    ThreadClient.include thread_client if thread_client
-    @ui.extend ui if ui
-  end
-
   # manual configuration methods
 
   def self.add_line_breakpoint file, line, **kw