Install Yarn

[yahonda]

This pull request installs yarn 1.12.1 to support Rails 6 Webpacker.

Installation guide:

https://yarnpkg.com/en/docs/install#debian-stable

Steps to reproduce:

> vagrant up
> vagrant ssh
$ git clone https://github.com/rails/rails.git
$ cd railties
$ bundle install
$ bundle exec rake test

Actual result without this pull request:

vagrant@rails-dev-box:~/rails/railties$ bundle exec rake test
rake aborted!
Errno::ENOENT: No such file or directory - yarn
/home/vagrant/rails/railties/test/isolation/abstract_unit.rb:478:in ``'
/home/vagrant/rails/railties/test/isolation/abstract_unit.rb:478:in `block (2 levels) in <top (required)>'
/home/vagrant/rails/railties/test/isolation/abstract_unit.rb:477:in `chdir'
/home/vagrant/rails/railties/test/isolation/abstract_unit.rb:477:in `block in <top (required)>'
/home/vagrant/rails/railties/test/isolation/abstract_unit.rb:470:in `initialize'
/home/vagrant/rails/railties/test/isolation/abstract_unit.rb:470:in `new'
/home/vagrant/rails/railties/test/isolation/abstract_unit.rb:470:in `<top (required)>'
/home/vagrant/rails/railties/Rakefile:32:in `require_relative'
/home/vagrant/rails/railties/Rakefile:32:in `block (2 levels) in <top (required)>'
Tasks: TOP => test => test:isolated
(See full trace by running task with --trace)
vagrant@rails-dev-box:~/rails/railties$ pwd
/home/vagrant/rails/railties
vagrant@rails-dev-box:~/rails/railties$

Note:

yarn pkg available from the Ubuntu distribution is version 0.32, which does not satisfy the Webpacker requirement.

$ bundle exec rake test
00h00m00s 0/0: : ERROR: [Errno 2] No such file or directory: 'build'
Webpacker requires Yarn >= 1.0.0 and you are using 0.32
Please upgrade Yarn https://yarnpkg.com/lang/en/docs/install/
00h00m00s 0/0: : ERROR: [Errno 2] No such file or directory: 'add'
Usage: yarn [options]

yarn: error: no such option: --tilde

Installed yarn version by this pull request:

$  yarn -v
1.12.1

[yahonda]

Closed via 2c1c2d5

[fxn]

Oh sorry! I missed this PR and noticed Yarn was missing yesterday while testing some stuff. Thanks for the patch anyway ❤️.