feat: Add IsRunning() method to query scheduler state

[CybotTM]

Summary

Add a method to query whether the scheduler is currently running.

Upstream Reference

• PR: feat(cron): adding the IsRunning function robfig/cron#495 - IsRunning() method

Problem

Currently there's no way to check if a scheduler is running:

c := cron.New()
c.Start()

// Later in code...
// How do I know if c is running?

Proposed API

c := cron.New()
fmt.Println(c.IsRunning()) // false

c.Start()
fmt.Println(c.IsRunning()) // true

c.Stop()
fmt.Println(c.IsRunning()) // false

Use Cases

• Conditional start (avoid double-start)
• Health checks and monitoring
• Graceful shutdown coordination
• Debugging and logging

Priority

Lower - Small quality-of-life improvement.

Implementation Notes

Simple implementation using existing running field. Should be thread-safe.