fix(clerk-js): Handle single part hostnames in getCookieDomain()

[brkalow]

Description

Handle single-part hostnames when determining the cookie domain. This is necessary for custom hosts defined locally in /etc/hosts, e.g.

127.0.0.1 local-app

Checklist

• [x] npm test runs as expected.
• [ ] npm run build runs as expected.
• [ ] (If applicable) JSDoc comments have been added or updated for any package exports
• [ ] (If applicable) Documentation has been updated

Type of change

• [x] 🐛 Bug fix
• [ ] 🌟 New feature
• [ ] 🔨 Breaking change
• [ ] 📖 Refactoring / dependency upgrade / documentation
• [ ] other: