title: Svelte bind is not working when customElement: true is set
body: <p>I want to build a custom element using Svelte.</p>
  
  <p>Thus in <code>rollup.config.js</code> I set <code>customElement: true</code>, and then I have to use the  to refer to my child components.</p>
  
  <p>But I found that in this way, the <code>bind</code> will not work. Here is the code example</p>
  
  <p>HelloWorld.svelte (child)</p>
  
  <pre class="lang-js prettyprint-override"><code>&lt;script&gt;
      import Hello from './components/Hello'
      import World from './components/World'
      export let value;
  &lt;/script&gt;
  
  &lt;svelte:options tag={'x-app-helloworld'}/&gt;
  &lt;input type="text" bind:value={value} &gt;
  
  &lt;input&gt;
  &lt;x-app-hello /&gt;
  &lt;x-app-world /&gt;
  </code></pre>
  
  <p>App.svslte(parent) part of it.</p>
  
  <pre class="lang-js prettyprint-override"><code>&lt;x-app-helloworld bind:value={value}/&gt;
  
  </code></pre>
  
  <p>Then the parent will show an error: <code>'value' is not a valid binding on &lt;x-app-helloworld&gt; elements.</code></p>
  
  <p>How could I solve this <code>bind</code> problem?</p>