Improve types to reflect optional fields

[louisjrdev]

I was using this module and noticed that there are some required fields in the types that are actuall optional on the API for the checkout (most of the fields are optional)

So this is just an update to relect that.

Here is a quick link to the docs for the checkout if you want to confirm:
https://docs.lemonsqueezy.com/api/checkouts#create-a-checkout

[louisjrdev]

Oh nevermind, its been sorted in an open PR #20