feat: support multiple handlers for a single subscription

[jonahsnider]

🔗 Linked issue

Closes #65

❓ Type of change

• [ ] 🐞 Bug fix (a non-breaking change that fixes an issue)
• [ ] 👌 Enhancement (improving an existing functionality like performance)
• [x] ✨ New feature (a non-breaking change that adds functionality)
• [ ] ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Allows adding multiple handlers to a single channel subscription. The old API will still work. A new API was added for removing specific handlers without fully unsubscribing from the channel:

const handler1 = () => { /* ... */ };

// start subscription
await redis.subscribe('users/123', handler1);

const handler2 = () => { /* ... */ }; 

// use the existing subscription
await redis.subscribe('users/123', handler2);

// remove handler1, keep subscription alive
await redis.unsubscribe('users/123', handler1);

// no more handlers for channel, close subscription
await redis.unsubscribe('users/123', handler2);

To avoid introducing any breaking changes, the errors in errors.ts were marked @deprecated, since they are no longer relevant with the introduction of this feature.

📝 Checklist

• [x] I have linked an issue or discussion.
• [ ] I have updated the documentation accordingly.