Use Reline for fallback of ext/readline
Ruby loses readline standard library when the system doesn't have libreadline during Ruby installation, At https://bugs.ruby-lang.org/issues/11084, hsbt (Hiroshi SHIBATA) proposed pure Ruby implementation. But I think that rb-readline is not good for Ruby standard library and I described it at http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-core/86213. The important reason for it personally is that rb-readline doesn't have vi mode.
So I'm implementing Reline for a new standard library as GNU Readline and existing ext/readline compatible. I can maintain it.
I don't think that Reline should replace ext/readline fully, it's just for fallback. But it's important for Ruby.
IRB is improved with Reline and RDoc
Reline is a readline stdlib compatible library. It also supports
multiline input. IRB is improved with Reline and supports multiline.
Besides, supports showing documents when completed.