In Vue 3's custom elements interop, we no longer process `is` usage on
known native elements as component casting. (ref:
https://v3.vuejs.org/guide/migration/custom-elements-interop.html)
This introduced the need for `v-is`. However, since it is a directive,
its value is considered a JavaScript expression. This makes it awkward
to use (e.g. `v-is="'foo'"`) when majority of casting is non-dynamic,
and also hinders static analysis when casting to built-in Vue
components, e.g. transition-group.
This commit adds the ability to cast a native element to a Vue component
by simply adding a `vue:` prefix:
```html
<button is="vue:my-button"></button>
<ul is="vue:transition-group" tag="ul"></ul>
```
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