Allow commands to run via yarn

[y-yagi]

Currently, commands(webpack and webpacker-dev-server) need to exist under the node_modules/.bin directory.

But when Yarn Plug'n'Play using, node_modules/.bin is not created.
Ref: https://github.com/yarnpkg/rfcs/blob/master/accepted/0000-plug-an-play.md

In this case, I think it is better to run the commands via yarn.

For this reason, I added new config key, and if it is specified, run commands via yarn.

I'm not sure if it is appropriate to set this behavior in the configuration file.
If env is appropriate, I will update it. Thanks!

[gauravtiwari]

@y-yagi Thanks for this. Looks great, I am not sure about config option though therefore, I am wondering would it make sense to check file presence and fallback to yarn? (do this implicitely)

if File.exist?('node_modules/.bin/webpack')
else 
  # try yarn
end

[y-yagi]

Thanks for your feedback!
I added config to think that it is confusing when using Yarn Plug'n'Play to break by whether node_modules exists or not.
(This is because it predicts that node_modules will remain in many cases when migrating to Yarn Plug'n'Play)

But this is probably the only problem in the beginning. Otherwise, it makes more sense to check if the file exists.
I updated to fix that check node_modules exists or not.

[y-yagi]

Because of the effect that BigDecimal.new removed, the build of 4.2 + ruby-head is an fails. This fixes with #1825.

[gauravtiwari]

Merged #1825 thanks for the update

[gauravtiwari]

If you could rebase with the master it should pass 👍

[y-yagi]

Rebased!

[gauravtiwari]

Great, merged 👍 🍰

[y-yagi]

Thanks!