title: How to use jQuery with rails webpacker 3
body: <p>I generate a new rails app :
  <code>
  rails new titi --webpack=vue
  </code></p>
  
  <p>and would like to use jQuery (or other libs like popper, vue-resource...).</p>
  
  <p>I tried to <code>yarn add jquery</code>which was fine, but I don't have access to jQuery in my JavaScript code.</p>
  
  <p>In previous Webpacker gem, there was much more conf and I had to write this in <code>shared.js</code> :</p>
  
  <pre><code>module.exports = {
      ...
      plugins: [
        ...
        new webpack.ProvidePlugin({
            $: 'jquery',
            jQuery: 'jquery',
      ]
      ...
      resolve: {
        extensions: settings.extensions,
        modules: [
         resolve(settings.source_path),
          'node_modules'
        ],
        alias: {
          jquery: "jquery/src/jquery",
          vue: "vue/dist/vue.js",
          vue_resource: "vue-resource/dist/vue-resource",
      }
  }
  </code></pre>
  
  <p>What is the cleanest way to include libraries in current Webpacker version ? Something in <code>config/webpacker.yml</code>? </p>