Fix create checkout logic

[NuroDev]

What's been changed?

• Updated LemonsqueezyOptions to take an optional generic to set the type of the data property
• Fixed multiple checkout types to be optional rather than required
• Added new LemonsqueezyCheckoutPreview type
• Updated LemonsqueezyCheckout to add new preview property
  • Needs documentation comment adding
• Updated CreateCheckoutOptions type to manually write out / type properties
• Added new CreateCheckoutBody type to type the POST request body
• Improved core error handling

What still needs to be done

• [x] Update checkout tests
  • [x] Creating a checkout
  • [x] Retrieve a checkout using the newly created one above