Require a local run.rb file on ruby -run

[sdwolfz]

Crazy idea, I know, but hear me out!

This is to allow more functinality to be built on top of what lib/un.rb
provides. With this change you can have a run.rb file locally with
arbitrary ruby code...

puts 'Hello!'

...which will be executed when you do ruby -run from that directory.

You should aslo be able to execut functions...

def greet
  puts 'Hello!'
end

...using the -e flag: ruby -run -e greet

Best of all, you can reuse the setup provided in un.rb to handle
extra flags:

def greet
  setup('w') do |argv, options|
    puts "Hello #{argv[0]}"
  end
end

...which gets executed with: ruby -run -e greet -- -w World

The possibilities are endless!

[hsbt]

https://bugs.ruby-lang.org/issues/17328#change-88529