Request to Implement Unsubscribe Functionality in the Mitt Framework

### Request to Implement Unsubscribe Functionality in the Mitt Framework

**Description:**
The current version of the Mitt framework lacks an `unsubscribe` feature, making it inconvenient to manage event listeners, especially when components are unmounted or no longer need to listen for events.

**Suggested Feature:**
Implement an `unsubscribe` method that allows users to easily remove specific event listeners.

**Example:**
```javascript
// Current usage in Mitt
const mitt = require('mitt');
const emitter = mitt();

const handler = () => console.log('Event triggered');
emitter.on('event', handler);
emitter.off('event', handler);
```
```javascript
// After adding the unsubscribe feature
const mitt = require('mitt');
const emitter = mitt();

// Proposed usage
const unsubscribe = emitter.on('event', () => console.log('Event triggered'));
unsubscribe(); // Now we can easily unsubscribe
```

**Conclusion:**
Adding the `unsubscribe` functionality would greatly improve the usability of the Mitt framework and align it with best practices in event handling. Thank you for considering this enhancement!

El issue no menciona archivos ni pruebas. Comienza por la API pública de emitter de Mitt y compara el comportamiento existente de on/off con el valor de retorno propuesto; confirma las expectativas de compatibilidad, añade cobertura para el uso de unsubscribe y verifica que el ejemplo documentado funcione.

Escrito por el modelo de indexación a partir del texto del issue.