Add missing resources and fix bugs

[gerbal]

This PR has the cumulative changes we have made to this library to support missing resource types and fix some unexpected behavior, specifically the formatting of nested queries is not handled correctly.

In summary the changes in this PR are:

• run mix format
• add ex_check (and thereby Credo)
• add resources:
  • Coupon
  • CouponSet
  • CouponCode
  • Item
  • ItemPrice
  • PaymentIntent
  • PaymentSource
• add basic documentation of all modules
• refactor Resource for readability, use pipe chains for cleaner code
• fix formatting of nested parameters to exactly match Chargebee's official library behavior.

[gerbal]

@NicolasMarlier any interest in merging this or should I fork my own hex package?

[ringofhealth]

I think its better if you fork this and build your own hex. This is a huge improvement on the existing repo