Support Arduino Uno Wifi Rev2 (ATmega4809)

[icedream]

While there is already support for Arduino Uno Rev3 that I've been using, the other Arduino device I own which is a Uno Wifi Rev2 seems to be a different enough from it that I have been unsuccessful in getting tinygo-compiled code to boot on it.

Device information: https://docs.arduino.cc/hardware/uno-wifi-rev2

Possibly related ticket since the ATmega4809 has been mentioned there before: #2898