fix: checkout types & feat: customer module

[carstenlebek]

Made optional fields optional, added subscription_preview and fixed the type for custom

[NuroDev]

Thanks for the work on this @carstenlebek 🙏. I am going to have a full look through the changes shortly.

On quick glance I can see a lot of code formatting changes. I have just update main to include a Prettier config & a format command to fix any formatting issues. Please pull the changes & run the format command so everything can stay consistant.

[carstenlebek]

Sorry, my bad 🤦🏼‍♂️ fixed the formatting issues

[carstenlebek]

All done 🥳