Allow optional `checkout_data`, `checkout_options`, & `product_options` field keys.

[kaleabmelkie]

Currently, all the fields of LemonsqueezyCheckoutData, LemonsqueezyCheckoutOptions, & LemonsqueezyProductOptions are required in TypeScript.

When calling createCheckout, we get a type error unless all the fields are provided for these objects:

I believe the fix should be as simple as making the fields of the three type above optional, right?

I made them optional in my local node_modules folder and it seemed to work (locally). I'm just not sure if any of these fields need to be required (not optional):

[NuroDev]

Changes for this had already been made recently to improve this but weren't perfect.

I have just pushed a new change that should fix this.

I will publish a new version with correctly functioning create checkout logic soon.

[kaleabmelkie]

Alright, thanks!