Avoid overriding user's irb_name setting in debugger integration

Instead of always setting `irb_name` to `irb:rdbg`, it should respect
the user's setting and only append `:rdbg` to it.

lib/irb/debug.rb

@@ -73,7 +73,7 @@ def configure_irb_for_debugger(irb)
         require 'irb/debug/ui'
         IRB.instance_variable_set(:@debugger_irb, irb)
         irb.context.with_debugger = true
-        irb.context.irb_name = "irb:rdbg"
+        irb.context.irb_name += ":rdbg"
       end
 
       def binding_irb?

test/irb/test_debug_cmd.rb

@@ -289,6 +289,30 @@ def test_prompt_line_number_continues
       assert_match(/irb:rdbg\(main\):005> next/, output)
     end
 
+    def test_prompt_irb_name_is_kept
+      @irbrc = Tempfile.new('irbrc')
+      @irbrc.write <<~RUBY
+        IRB.conf[:IRB_NAME] = "foo"
+      RUBY
+      @irbrc.close
+      @envs['IRBRC'] = @irbrc.path
+
+      write_ruby <<~'ruby'
+        binding.irb
+        puts "Hello"
+      ruby
+
+      output = run_ruby_file do
+        type "next"
+        type "continue"
+      end
+
+      assert_match(/foo\(main\):001> next/, output)
+      assert_match(/foo:rdbg\(main\):002> continue/, output)
+    ensure
+      @irbrc&.unlink
+    end
+
     def test_irb_commands_are_available_after_moving_around_with_the_debugger
       write_ruby <<~'ruby'
         class Foo