title: npm start gives error `webpack-dev-server: not found` even if it is installed
body: <p>In package.json <code>devDependencies</code> I have webpack-dev-server installed:</p>
  
  <pre><code>"webpack-dev-server": "3.2.1"
  </code></pre>
  
  <p>but when I run the start script, which looks as follows:</p>
  
  <pre><code>"start": "webpack-dev-server --inline --config demo/webpack.config.js --port 9000 --mode development"
  </code></pre>
  
  <p>I get this error: </p>
  
  <pre><code>sh: 1: webpack-dev-server: not found
  npm ERR! file sh
  npm ERR! code ELIFECYCLE
  npm ERR! errno ENOENT
  npm ERR! syscall spawn
  npm ERR! your-app@2.0.5 start: `webpack-dev-server --inline --config demo/webpack.config.js --port 9000 --mode development`
  npm ERR! spawn ENOENT
  npm ERR! 
  npm ERR! Failed at the your-app@2.0.5 start script.
  npm ERR! This is probably not a problem with npm. There is likely additional logging output above.
  </code></pre>
  
  <p>What is the issue here?</p>