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-ssr/src/errors.ts

@@ -19,11 +19,11 @@ export function createSSRCompilerError(
 export const enum SSRErrorCodes {
   X_SSR_CUSTOM_DIRECTIVE_NO_TRANSFORM = DOMErrorCodes.__EXTEND_POINT__,
   X_SSR_UNSAFE_ATTR_NAME,
-  X_SSR_NO_PORTAL_TARGET
+  X_SSR_NO_TELEPORT_TARGET
 }
 
 export const SSRErrorMessages: { [code: number]: string } = {
   [SSRErrorCodes.X_SSR_CUSTOM_DIRECTIVE_NO_TRANSFORM]: `Custom directive is missing corresponding SSR transform and will be ignored.`,
   [SSRErrorCodes.X_SSR_UNSAFE_ATTR_NAME]: `Unsafe attribute name for SSR.`,
-  [SSRErrorCodes.X_SSR_NO_PORTAL_TARGET]: `No target prop on portal element.`
+  [SSRErrorCodes.X_SSR_NO_TELEPORT_TARGET]: `No target prop on teleport element.`
 }