title: $router is missing on the vue interface
body: <p>I'm using Vue with typescript (no babel) from the vue/cli.</p>
  
  <p>I want to use router: <code>this.$router</code> but I got error message: 
  <code>Property '$router' does not exist on type</code>.</p>
  
  <pre><code>import { Component, Prop, Vue } from 'vue-property-decorator';
  
  export default class App extends Vue {
    constructor() {
      super();
  
      this.$router. // &lt;- Property '$router' does not exist on type
    }
  }
  </code></pre>
  
  <p>Also I can't see any definition of <code>$router</code> on <code>vue</code> interface!</p>
  
  <p><a href="https://i.stack.imgur.com/L6lll.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/L6lll.png" alt="enter image description here"></a></p>
  
  <p>How to solve that please?</p>