feat: add planetscale support

[Gerbuuun]

🔗 Linked issue

#32

❓ Type of change

• [x] 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• [ ] 🐞 Bug fix (a non-breaking change that fixes an issue)
• [ ] 👌 Enhancement (improving an existing functionality like performance)
• [x] ✨ New feature (a non-breaking change that adds functionality)
• [ ] 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• [ ] ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

I have added a connector for @planetscale/database. It is mostly a copy of the Postgresql connector with some small changes.

Due to differences in SQL dialects I added a dialect option to the testConnector function. Using a switch statement, the correct query is used for the different dialects. Maybe there are better other options (suggestions)?

📝 Checklist

• [x] I have linked an issue or discussion.
• [x] I have updated the documentation accordingly.