Enable non-blocking process monitoring

[rsamoilov]

This pull request introduces a new method to support non-blocking process waiting in the fiber scheduler and adds tests to ensure its correct behavior.

Fiber Scheduler Enhancements:

• Added a process_wait method to lib/rage/fiber_scheduler.rb that waits for a process in a non-blocking way by delegating to a separate thread.

Testing Improvements:

• Added a new context for #process_wait in spec/fiber_scheduler_spec.rb with tests verifying:
  • Non-blocking behavior when waiting for processes.
  • Proper fiber resumption when the process finishes or is killed.