feat: mounting-portal component support mount to customized HTMLElement

[Ryqsky]

when the HTMLElement comes form other iframe so that window.document.querySelector is different and inoperative

[cihad]

@Ryqsky Is this can be an usage example:

<div id="app">
  <MountingPortal :mountTo="mountTo" name="source" append>
    <p>Content for the Target</p>
  </MountingPortal>
<div>

<aside id="widget" class="widget-sidebar">
  This Element is not controlled by our Vue-App,
  but we can create a <portal-target> here with <MountingPortal>.
</aside>

<script>
  new Vue({
    el: '#app',
    data() {
      return { mountTo: window['widget'] }
    },
  })
</script>

[Ryqsky]

@Ryqsky Is this can be an usage example:

<div id="app">
  <MountingPortal :mountTo="mountTo" name="source" append>
    <p>Content for the Target</p>
  </MountingPortal>
<div>

<aside id="widget" class="widget-sidebar">
  This Element is not controlled by our Vue-App,
  but we can create a <portal-target> here with <MountingPortal>.
</aside>

<script>
  new Vue({
    el: '#app',
    data() {
      return { mountTo: window['widget'] }
    },
  })
</script>

@cihad
Can you provide an online address in https://codesandbox.io/ ?
Thanks.