API: Changes to the voices list

When querying the list of the available voices, the result contains both your cloned voices and the system-wide voices provided by Speechify.

The latter have short distinctive names (like cliff or george), while the former have unique random alphanumeric IDs.

Previously to distinguish between the two Speechify API would prefix the cloned voice IDs with pvl: (short for Private Voices List). This was leaking the implementation detail irrelevant to our users, and the prefix was removed.

Instead, we now expose the new type property on the voice details object. It is "shared" for the Speechify system voices, and "personal" for the cloned voices that only you have access to.

The documentation was updated accordingly.