chore(playground): update syntax for github highlighting

[adamdehaven]

🔗 Linked issue

N/a

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality)
• ✨ 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

Since the playground is used as an example reference on GitHub, update the syntax to resolve proper syntax highlighting in the browser.

[farnabaz]

Thanks

[adamdehaven]

@farnabaz I also noticed the shiki setup example in the README where we're using MDC in Vue is no longer correct due to the external shiki changes.

I probably need to update the example (unless you have a drop-in, updated replacement?)