Deprecate predictable helper methods

[Pimm]

Deprecated methods/properties such as:

• payment.isOpen() (synonymous with payment.status == PaymentStatus.open)
• list.count (synonymous with list.length)
• error.getMessage() (synonymous with error.message)

Cons

1. Removing these methods/properties will be a breaking change.
2. Some of these methods/properties also exist in the PHP client, therefore removing them will also widen the gap between the two clients.

Pros

1. Consumers of an API expect every part of it to be significant. When a consumer sees that a list has both a length and a count, they might wonder "What is the difference between these two? Perhaps length is the number of elements in this array, where count is the total number of elements available?" Methods/properties with such simple synonyms can cause confusion.
2. Having a narrower API enhances discoverability. Without isOpen, isAuthorized, isExpired, isFailed, isPending, hasSequenceTypeFirst, and hasSequenceTypeRecurring, it's easier to find getCheckoutUrl and getRefunds. It's easier to find the needle if the haystack is smaller.
3. Having fewer roads leading to Rome promotes consistency in the code which uses this library.

[edorivai]

LGTM

[dsempel]

I agree this is better.