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/).
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