Add a warning message and --legacy of an alias of --singleline

lib/irb/context.rb

@@ -82,6 +82,12 @@ module IRB
         case use_multiline?
         when nil
           if STDIN.tty? && IRB.conf[:PROMPT_MODE] != :INF_RUBY && !use_singleline?
+            # Both of multiline mode and singleline mode aren't specified.
+            puts <<~EOM
+              This version of IRB is drastically different from the previous version.
+              If you hit any issues, you can use "irb --legacy" to run the old version.
+              If you want to just erase this message, please use "irb --multiline".
+            EOM
             @io = ReidlineInputMethod.new
           else
             @io = nil

lib/irb/init.rb

@@ -161,7 +161,7 @@ module IRB # :nodoc:
         end
       when "--noinspect"
         @CONF[:INSPECT_MODE] = false
-      when "--singleline", "--readline"
+      when "--singleline", "--readline", "--legacy"
         @CONF[:USE_SINGLELINE] = true
       when "--nosingleline", "--noreadline"
         @CONF[:USE_SINGLELINE] = false