[ruby/irb] Fix history-saving feature

(https://github.com/ruby/irb/pull/642)

* 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.

* Improve history-saving's ability check

Instead of checking the existence of `input_method_class::HISTORY`, we should
make every input method class declare if it supports history saving or not.

Since the default value is `false`, it shouldn't break any custom input method
that inherits from `IRB::InputMethod`.

https://github.com/ruby/irb/commit/aec7a5b3f5

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