title: Map is not correctly deserialized in NGXS and cannot be used
body: <p>NGXS is used for state management. There are many states that could be implemented as maps to write cleaner code and to improve performance. Currently there is a workaround where the keys are stored in the array as a field and via <code>findIndex</code> the arrays can be searched for the key. </p>
  
  <p>When I try to store a map via NGXS, the object is no map after deserialization and I cannot use the map functions like below:</p>
  
  <blockquote>
    <p>TypeError: member[index].map.get is not a function</p>
  </blockquote>
  
  <pre><code>@Action(ActionExample)
  @ImmutableContext()
  public action() {
    const state = getState();
    const entry = state.member[index].map.get(key);
  }
  
  </code></pre>
  
  <p>How can I use maps with NGXS?</p>