fix #5

• ensure best practices with the use of env and other underlying services by the framework
• name it as with-nextjs
• use prettier as done in https://github.com/polarsource/examples/tree/main/export-all-customers-to-csv
• install the latest version of Polar, shadcn/ui and Next.js
• maintain seperate API routes for each, Webhooks handler, Customer Portal handler and Checkout handler
• add an optional build-time script that validates if the access token used is valid for creating a checkout session in the relevant Polar mode (sandbox/production)
• create an installation guide like as in https://github.com/neondatabase/examples/tree/main/with-nestjs
• create the .env.example with the comments to the guides links of where the values can be obtained from
• share the deployed application to Vercel with the recording of all the 3 functionalities working