Add support for TX-Ultimate switch backlight

[FWMatt]

• Please explain your feature request in a one or two sentences.

The TX-Ultimate has a backlight. Expose this property to Homebridge.

• Is your feature request related to a problem? Please describe.

No.

• Any particular eWeLink devices that this relates to?

Sonoff TX-Ultimate

• Anything else?

Let me know if I can provide further diagnostics / output JSON.

I will sponsor if this feature is added.