[1.4.2] Make it more stable

What's changed

Changed

• Improved worker startup handling by returning state alongside worker PID or error, ensuring state consistency during worker creation and handling of failed worker starts. This prevents potential deadlocks when workers fail to start.
• Refactored worker down handling logic by extracting different scenarios into dedicated functions for better code readability and maintainability.

Fixed

• Fixed incorrect typespecs in Poolex.Private.IdleOverflowedWorkers.count/1 and Poolex.Private.IdleWorkers.count/1 (changed from neg_integer() to non_neg_integer()).
• Made Poolex.Private.Monitoring.remove/2 safer by not pattern matching on the Process.demonitor/1 return value.