Prune devDependencies for Yarn 2
Prune devDependencies for Yarn 2

Adds a custom Yarn 2 plugin to handle `devDependency` pruning.

Scalingo/nodejs-buildpack,heroku/heroku-buildpack-nodejs,Scalingo/nodejs-buildpack,Scalingo/nodejs-buildpack,heroku/heroku-buildpack-nodejs,heroku/heroku-buildpack-nodejs,Scalingo/nodejs-buildpack,heroku/heroku-buildpack-nodejs,heroku/heroku-buildpack-nodejs,Scalingo/nodejs-buildpack