FR: Zones support

Hello. Relatively new to using Bifrost, but it's been working well, and I really appreciate the effort! One thing I'm missing is support for zones though. I have some Zigbee groups like "Chandelier" and "Cascade Lamp" that have multiple bulbs in one fixture that act as one. It would be nice to be able to add zone groups in addition to room groups so I can control these devices. Zones would also be useful for configuring scenes for things like first floor/second floor or turning off all lights in the house.

I'm using the group_prefix option, so maybe this could be implemented with an additional zone_prefix option? So if I have
```
group_prefix: bifrost_
zone_prefixL zone_
```
a zigbee2mqtt group called "bifrost_zone_First Floor" would show up as a "First Floor" zone in the Hue app.

Please let me know what you think. Thank you!