title: Rails 6 fails to integrate slick-carousel
body: <p>I created a brand new project(rails 6.0.2.2). Now facing a problem
  <code>testing.js:2 Uncaught TypeError: $(...).slick is not a function</code> after integrate slick</p>
  
  <p>The way I integrate slick-carousel is:</p>
  
  <ul>
  <li>yarn add jquery(slick depends on it)</li>
  <li>yarn add slick-carousel</li>
  <li>in <code>app/javascript/packs/application.js</code>
  
  <ul>
  <li>require("jquery")</li>
  <li>require("slick-carousel")</li>
  <li>require("packs/testing")</li>
  </ul></li>
  <li>in <code>config/webpack/environment.js</code></li>
  </ul>
  
  <pre><code>environment.plugins.prepend('Provide',
  new webpack.ProvidePlugin({
   $: 'jquery/src/jquery',
   jQuery: 'jquery/src/jquery'
  })
  )
  </code></pre>
  
  <p>sources have both jquery and slick-carousel, but <code>$('.testing').slick()</code> gives me the above error</p>
  
  <p>package.json</p>
  
  <pre><code>"dependencies": {
      "@rails/actioncable": "^6.0.0",
      "@rails/activestorage": "^6.0.0",
      "@rails/ujs": "^6.0.0",
      "@rails/webpacker": "4.2.2",
      "bootstrap": "^4.4.1",
      "jquery": "^3.5.0",
      "slick-carousel": "^1.8.1",
      "turbolinks": "^5.2.0"
    },
  </code></pre>