Add combo LED modus

[domoritz]

I use an air gradient one for indoor air quality tracking. I like he LEDs as a quick status indicator for how things are. https://github.com/MallocArray/airgradient_esphome has a really cool/useful mode where the LEDs together indicate CO2, PM2.5 and VOC. The mode, called "combo" is defined in https://github.com/MallocArray/airgradient_esphome/blob/main/packages/led_combo.yaml.

# Combo LED bar
# Left 5 LEDs indicate CO2
# Middle 5 LEDs indicate PM2
# Far right LED indicates TVOC
# LED Fade setting fades out the bars on the sides at the set percentage each step

I really like this mode as it makes the LEDs more useful and readable from across the room. More motivation in MallocArray/airgradient_esphome#34 (comment).

Could you add a combo mode to the standard firmware? I'm specifically asking for home assistant.