title: On property change event handler
body: <p>In a VSCode <a href="https://github.com/attdona/riot-code" rel="nofollow noreferrer">extension</a> I'm looking for a way to react when the user modify an extension property defined in contributes.configuration section of package.json. </p>
  
  <p>Does it exists an event like onPropertyChange or some other way to register an event handler?</p>