Enum for `"pagination"` property in questionnaire

[nsenave]

The questionnaire class has a "pagination" property, which is a string.

➡️ Create an enum to prevent users of Lunatic-Model to insert invalid values in this filed.

[nsenave]

To do in a later releases:

1. Remove String methods for pagination, create Pagination getPagination() method and mark Pagination getPaginationEnum() as depracated.
2. Remove Pagination getPaginationEnum() method