hcho-density property as air quality sensor
Over last couple years Xiaomi has been adding proper HCHO (formaldehyde) to some of their upper end purifiers such as zhimi.airp.sa4 (4 MAX), and zhimi.airp.ua1a (Ultra).

While HomeKit doesn't have a formaldehyde specific category as it does for Carbon Dioxide, Monoxide, Nitrogen, Ozone and Sulphur Dioxide, formaldehyde is a VOC and HomeKit does support HMCharacteristicTypeVolatileOrganicCompoundDensity

While I'm able to pick up the mg/m3 value output of the hcho-density property, it's currently being displayed as lux within the Home app. One small thing to note is that within miot spec the value is in mg/m3 (milligrams) while the expected HomeKit unit is µg/m3 (micrograms).

Are we able to allow for 'native' output of the hcho-density property in HomeKit where it would appear as a TVOC sensor as does Aqara's TVOC unit

![Aqara-TVOC-AQI-settings](https://github.com/merdok/homebridge-miot/assets/2682413/f98854c3-c365-4eec-90c8-0cfbb33c67ab)

https://homekitnews.com/2021/07/16/aqara-tvoc-sensor-review/