add Random.random_number

[ahorek]

random_number is just an alias to rand

fixes

Random.new.random_number

Random.random_number (and SecureRandom.bytes #5773) is actually a ruby 2.6 feature, should I keep it or move it to 2.6 branch?

[ahorek]

looking at the source again, it looks like SecureRandom defines this method dynamically after
require 'securerandom'
https://github.com/ruby/ruby/blob/master/random.c#L1567

[ahorek]

SecureRandom should automatically extend the original Random class https://github.com/ruby/ruby/blob/c5eb24349a4535948514fe765c3ddb0628d81004/lib/securerandom.rb#L135

do we wanna support extending Random via Random::Formatter? anyway this PR is wrong, so closing for now.