Include charging state for Device 14 - Battery messages

[raphaelcherney]

When we get information about the battery state, it would be helpful to know whether it is actively being charged. Root turns off when charging due some toy-specific requirements, but that is not the case for Create 3 or other potential robots using this protocol. I propose we add a charging status byte for Device 14 messages. Something like:
0 = no charger connected (backward compatible with Root)
1 = charging
2 = charging complete

In the future, we could even consider additional states like charging error, fast charging, slow charging, etc. that would be good for the controller to know.