Add Webpacker

Most of this is automatically generated by the rake task: `bundle exec rake webpacker:install`

Docs: https://github.com/rails/webpacker/tree/5-x-stable

bin/webpack-dev-server

@@ -0,0 +1,18 @@
+#!/usr/bin/env ruby
+
+ENV["RAILS_ENV"] ||= ENV["RACK_ENV"] || "development"
+ENV["NODE_ENV"]  ||= "development"
+
+require "pathname"
+ENV["BUNDLE_GEMFILE"] ||= File.expand_path("../../Gemfile",
+  Pathname.new(__FILE__).realpath)
+
+require "bundler/setup"
+
+require "webpacker"
+require "webpacker/dev_server_runner"
+
+APP_ROOT = File.expand_path("..", __dir__)
+Dir.chdir(APP_ROOT) do
+  Webpacker::DevServerRunner.run(ARGV)
+end