Define RelineInputMethod::HISTORY

The HistorySavingAbility module doesn't do anything if the input method
class doesn't define HISTORY.

- https://github.com/ruby/irb/blob/3ac96be660bf052902fb4e532c7a46cf294b71eb/lib/irb/history.rb#L10
- https://github.com/ruby/irb/blob/3ac96be660bf052902fb4e532c7a46cf294b71eb/lib/irb/history.rb#L34

This patch defines RelineInputMethod::HISTORY to avoid this.

lib/irb/input-method.rb

@@ -255,6 +255,7 @@ def inspect
   end
 
   class RelineInputMethod < InputMethod
+    HISTORY = Reline::HISTORY
     # Creates a new input method object using Reline
     def initialize
       IRB.__send__(:set_encoding, Reline.encoding_system_needs.name, override: false)