Fix DAP's completion request with new IRB's completor

In IRB 1.8.2, the completor's design and API has been changed and thus
would break DAP server's completion request. This commit fixes the
issue by introducing a `Completor` class that abstracts away the
differences between the old and new completor.

lib/debug/completor.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+require "irb/completion"
+
+module DEBUGGER__
+  class Completor
+    class << self
+      # old IRB completion API
+      if defined?(IRB::InputCompletor)
+        def retrieve_completion_data(input, binding)
+          IRB::InputCompletor.retrieve_completion_data(input, bind: binding).compact
+        end
+      else
+        COMPLETOR = IRB::RegexpCompletor.new
+
+        def retrieve_completion_data(input, binding)
+          COMPLETOR.retrieve_completion_data(input, bind: binding, doc_namespace: false).compact
+        end
+      end
+    end
+  end
+end

lib/debug/server_dap.rb

@@ -5,6 +5,8 @@
 require 'tmpdir'
 require 'fileutils'
 
+require_relative 'completor'
+
 module DEBUGGER__
   module UI_DAP
     SHOW_PROTOCOL = ENV['DEBUG_DAP_SHOW_PROTOCOL'] == '1' || ENV['RUBY_DEBUG_DAP_SHOW_PROTOCOL'] == '1'
@@ -980,7 +982,7 @@ def process_dap args
         frame = get_frame(fid)
 
         if (b = frame&.binding) && word = text&.split(/[\s\{]/)&.last
-          words = IRB::InputCompletor::retrieve_completion_data(word, bind: b).compact
+          words = Completor.retrieve_completion_data(word, b)
         end
 
         event! :protocol_result, :completions, req, targets: (words || []).map{|phrase|