chore(examples): Convert api-routes-cors example to TypeScript

[maxproske]

Convert api-routes-cors example to TypeScript to match Contribution docs.

• Update CORS configuration to match the middleware Next.js docs
• Allow POST requests as per instructions in index.tsx: "make a POST / GET / OPTIONS request to /api/cors"

Documentation / Examples

• Make sure the linting passes by running pnpm lint
• The examples guidelines are followed from our contributing doc

[balazsorban44]

Thanks!