Page the output in irb:rdbg sessions too (#1043)

IRB started to page its evaluation output and it became a useful feature
for users. However, in `irb:rdbg` sessions, the output is not paged so
the sudden change in behavior is surprising and inconvenient.

This commit makes `irb:rdbg` sessions page the output of the debugger
too.

lib/irb/debug/ui.rb

@@ -45,9 +45,7 @@ def puts str = nil
             $stdout.puts line.chomp
           }
         when String
-          str.each_line{|line|
-            $stdout.puts line.chomp
-          }
+          Pager.page_content(str, retain_content: true)
         when nil
           $stdout.puts
         end