feat: Add stop() method to Server for graceful shutdown and IOLoop control

[maboloshi]

This PR adds a stop() method to the livereload.Server class, enabling proactive and graceful shutdown of the development server—particularly important for threaded integrations such as Sublime Text plugins or testing environments.

✨ Key Changes:

• Server.stop():

  • Closes all registered HTTP server instances (created via .listen())
  • Checks if the underlying Tornado IOLoop (which wraps asyncio) is still running via self.ioloop.asyncio_loop.is_running()
  • Uses self.ioloop.add_callback(self.ioloop.stop) to safely stop the loop from any thread

• Thread-safe and robust: Designed to work reliably even when livereload is used in non-main threads

• No breaking changes: This is an additive change; existing behavior remains untouched unless .stop() is explicitly called

✅ Benefits:

• Enables Sublime Text plugins (or other threaded environments) to cleanly terminate livereload without lingering ports or resources
• Prevents issues like OSError: address already in use when restarting livereload
• Opens up the ability to build testable and restartable development tools on top of livereload