Use class-variance-authority in Next.js example

[joe-bell]

Description

Use class-variance-authority in Next.js example (@joebell/cx is deprecated)

Additional context

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Follow the Style Guide.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Updated
plaiceholder-docs	✅ Ready (Inspect)	Visit Preview	Jun 18, 2022 at 6:05PM (UTC)
plaiceholder-with-11ty	✅ Ready (Inspect)	Visit Preview	Jun 18, 2022 at 6:05PM (UTC)
plaiceholder-with-next	✅ Ready (Inspect)	Visit Preview	Jun 18, 2022 at 6:05PM (UTC)