title: pinia use state instead of this in actions
body: <p>In Vue3 compisition API，I don't usually use <code>this</code></p>
  <p>But pinia example is:</p>
  <pre><code>increment() {
    this.counter++
  },
  </code></pre>
  <p>Dont want use <code>this</code> in action.Has any suggest?</p>