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/compiler-core/__tests__/transforms/transformElement.spec.ts

@@ -11,7 +11,7 @@ import {
   RESOLVE_DIRECTIVE,
   TO_HANDLERS,
   helperNameMap,
-  PORTAL,
+  TELEPORT,
   RESOLVE_DYNAMIC_COMPONENT,
   SUSPENSE,
   KEEP_ALIVE,
@@ -272,16 +272,16 @@ describe('compiler: element transform', () => {
     })
   })
 
-  test('should handle <Portal> with normal children', () => {
+  test('should handle <Teleport> with normal children', () => {
     function assert(tag: string) {
       const { root, node } = parseWithElementTransform(
         `<${tag} target="#foo"><span /></${tag}>`
       )
       expect(root.components.length).toBe(0)
-      expect(root.helpers).toContain(PORTAL)
+      expect(root.helpers).toContain(TELEPORT)
 
       expect(node).toMatchObject({
-        tag: PORTAL,
+        tag: TELEPORT,
         props: createObjectMatcher({
           target: '#foo'
         }),
@@ -298,8 +298,8 @@ describe('compiler: element transform', () => {
       })
     }
 
-    assert(`portal`)
-    assert(`Portal`)
+    assert(`teleport`)
+    assert(`Teleport`)
   })
 
   test('should handle <Suspense>', () => {