Fallback to Reline when require 'readline' fails

[tompng]

Require readline may fail because it is a bundled gem in 3.5.0.dev.

[tompng]

Adds ReadlineInputMethod::Readline = ::Readline or ::Reline
because we don't want to pollute the top level by adding constant ::Readline when require 'readline' fails.

[tompng]

To use GNU readline, now we need to add both gem 'readline' and gem 'readline-ext' to Gemfile.

Maybe we can require "readline.#{RbConfig::CONFIG['DLEXT']}" instead of 'readline' (https://github.com/ruby/readline/blob/master/lib/readline.rb)
Then users only need to add readline-ext to Gemfile just as it has been before.

[st0012]

Is there any feature that readline does better than reline? Does it make sense to still allow both types of input methods going forward 🤔

[tompng]

I didn't thought of that. Maybe it's time to use Reline also in "--singleline", "--readline", "--legacy" mode.

(Off topic: irb --readline was useful for me to check the difference between Reline and Readline while developing Reline)