Support setting the brightness on iOS

[davidbbs]

Support command_screen_brightness_level for iOS.

Use case: I would like to dim the screen on our bedroom iPad at night but have it brighter during the day. I can do this via an automation if the iOS app supports setting brightness.

Apple docs for UIKit:

        UIScreen.main.brightness = CGFloat(brightness)

(I can kind of see where to add this in the code, but I'm not sure whether the "LegacyEvent" code is still the right place.)

[rolandwellinger]

I support this!