fix(Lookbook): Enable previews for alert and broadcast message

Enables the ViewComponent previews of the Alert and Broadcast message components.

Merge this only after https://gitlab.com/gitlab-org/frontend/playground/lookbook-app/-/merge_requests/5 ✅

How to review

• In the deployed review app, go to these pages and see the Lookbook preview controls:
  • https://design-gitlab-com-gitlab-org-gitlab-services-f6beaf2fb7fd4cad76.gitlab.io/pages-review-mr-4-cpsdd5/components/alert/lookbook
  • https://design-gitlab-com-gitlab-org-gitlab-services-f6beaf2fb7fd4cad76.gitlab.io/pages-review-mr-4-cpsdd5/components/broadcast-message/lookbook

Before	After