an implementation of Reactive Extensions / rxjs for Perl

Changes for v6.28.0 - 2023-09-28T18:45:00Z

• removed op_group_by operator, because it was too buggy. (If anyone has a good grasp of how it works/what it should do, feel free to contact me)
• removed EXPERIMENTAL status of the two promise functions, since rxjs v7 has been released