refactor: rename <portal> to <teleport>

BREAKING CHANGE: `<portal>` has been renamed to `<teleport>`.

    `target` prop is also renmaed to `to`, so the new usage will be:

    ```html
    <Teleport to="#modal-layer" :disabled="isMobile">
      <div class="modal">
        hello
      </div>
    </Teleport>
    ```

    The primary reason for the renaming is to avoid potential naming
    conflict with [native portals](https://wicg.github.io/portals/).

packages/vue/examples/transition/modal.html

@@ -45,14 +45,14 @@ const Modal = {
 <!-- app -->
 <div id="app">
   <button id="show-modal" @click="showModal = true">Show Modal</button>
-  <portal target="#modal-container">
+  <teleport to="#modal-container">
     <!-- use the modal component, pass in the prop -->
     <modal :show="showModal" @close="showModal = false">
       <template #header>
         <h3>custom header</h3>
       </template>
     </modal>
-  </portal>
+  </teleport>
 </div>
 
 <script>