title: Webpack Dev Server Config - contentBase not working in latest version
body: <p>When I upgrade webpack to 4.0.0-beta.3 and run <code>npx webpack serve</code> I get this error:</p>
  <pre><code>[webpack-cli] Invalid configuration object. Object has been initialized using a configuration object that does not match the API schema.
   - configuration has an unknown property 'contentBase'. These properties are valid:
     object { bonjour?, client?, compress?, devMiddleware?, firewall?, headers?, historyApiFallback?, host?, hot?, http2?, https?, liveReload?, onAfterSetupMiddleware?, onBeforeSetupMiddleware?, onListening?, open?, port?, proxy?, public?, setupExitSignals?, static?, transportMode?, watchFiles? }
  </code></pre>
  <p>This is my webpack.config.js that works in 3.11.2:</p>
  <pre><code>const path = require('path');
  const ArcGISPlugin = require(&quot;@arcgis/webpack-plugin&quot;);
  
  module.exports = {
    mode: 'development',
    entry: {
        main: './app/main.js'
    },
    plugins: [
        new ArcGISPlugin()
    ],
    devServer: {
        contentBase: './'
    }
  }
  </code></pre>
  <p>my devDependencies from package.json:</p>
  <pre><code>  &quot;devDependencies&quot;: {
      &quot;@arcgis/webpack-plugin&quot;: &quot;^4.18.0&quot;,
      &quot;dojo-typings&quot;: &quot;^1.11.11&quot;,
      &quot;webpack-cli&quot;: &quot;^4.7.2&quot;,
      &quot;webpack-dev-server&quot;: &quot;^4.0.0-beta.3&quot;
  </code></pre>
  <p>How do I need to update my config to get the latest version working? When I take out the dev server object, the server will run, but serve content out of ./public which doesn't exist.</p>
  <p>I'm new to webpack, so I'm not yet familiar with the application, config, and requirements.</p>