Hint debugger command in irb:rdbg session (#768)

When user enters irb:rdbg session, they don't get the same hint that the
`debug` gem provides, like

```
(rdbg) n    # next command
```

This means that users may accidentally execute commands when they want to
retrieve the value of a variable.

So this commit adds a Reline output modifier to add a simiar hint:

```
irb:rdbg(main):002> n # debug command
```

It is not exactly the same as `debug`'s because in this case the importance
is to help users distinguish between value evaluation and debugger command
execution.

lib/irb/debug.rb

@@ -57,6 +57,24 @@ def DEBUGGER__.capture_frames(*args)
           DEBUGGER__::ThreadClient.prepend(SkipPathHelperForIRB)
         end
 
+        if !@output_modifier_defined && !DEBUGGER__::CONFIG[:no_hint]
+          irb_output_modifier_proc = Reline.output_modifier_proc
+
+          Reline.output_modifier_proc = proc do |output, complete:|
+            unless output.strip.empty?
+              cmd = output.split(/\s/, 2).first
+
+              if DEBUGGER__.commands.key?(cmd)
+                output = output.sub(/\n$/, " # debug command\n")
+              end
+            end
+
+            irb_output_modifier_proc.call(output, complete: complete)
+          end
+
+          @output_modifier_defined = true
+        end
+
         true
       end
 

test/irb/yamatanooroti/test_rendering.rb

@@ -379,6 +379,48 @@ def test_pager_page_content_doesnt_page_output_when_it_fits_in_the_screen
     assert_match(/foobar/, screen)
   end
 
+  def test_debug_integration_hints_debugger_commands
+    write_irbrc <<~'LINES'
+      IRB.conf[:USE_COLORIZE] = false
+    LINES
+    script = Tempfile.create(["debug", ".rb"])
+    script.write <<~RUBY
+      puts 'start IRB'
+      binding.irb
+    RUBY
+    script.close
+    start_terminal(40, 80, %W{ruby -I#{@pwd}/lib #{script.to_path}}, startup_message: 'start IRB')
+    write("debug\n")
+    write("n")
+    close
+
+    screen = result.join("\n").sub(/\n*\z/, "\n")
+    assert_include(screen, "irb:rdbg(main):002> n # debug command")
+  ensure
+    File.unlink(script) if script
+  end
+
+  def test_debug_integration_doesnt_hint_non_debugger_commands
+    write_irbrc <<~'LINES'
+      IRB.conf[:USE_COLORIZE] = false
+    LINES
+    script = Tempfile.create(["debug", ".rb"])
+    script.write <<~RUBY
+      puts 'start IRB'
+      binding.irb
+    RUBY
+    script.close
+    start_terminal(40, 80, %W{ruby -I#{@pwd}/lib #{script.to_path}}, startup_message: 'start IRB')
+    write("debug\n")
+    write("foo")
+    close
+
+    screen = result.join("\n").sub(/\n*\z/, "\n")
+    assert_include(screen, "irb:rdbg(main):002> foo\n")
+  ensure
+    File.unlink(script) if script
+  end
+
   private
 
   def write_irbrc(content)