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/shared/src/shapeFlags.ts

@@ -5,7 +5,7 @@ export const enum ShapeFlags {
   TEXT_CHILDREN = 1 << 3,
   ARRAY_CHILDREN = 1 << 4,
   SLOTS_CHILDREN = 1 << 5,
-  PORTAL = 1 << 6,
+  TELEPORT = 1 << 6,
   SUSPENSE = 1 << 7,
   COMPONENT_SHOULD_KEEP_ALIVE = 1 << 8,
   COMPONENT_KEPT_ALIVE = 1 << 9,