targets: rename arduino target to arduino-uno

Rename the "arduino" target to "arduino-uno" to better reflect the
board it represents. The "arduino" name is kept as an alias that
inherits from "arduino-uno" for backward compatibility.

- Rename targets/arduino.json to targets/arduino-uno.json
- Create targets/arduino.json as alias inheriting from arduino-uno
- Rename board_arduino.go to board_arduino_uno.go
- Update build tag from "arduino" to "arduino_uno"
- Rename corresponding example files

Signed-off-by: deadprogram <ron@hybridgroup.com>

src/machine/board_arduino_uno.go

@@ -1,4 +1,4 @@
-//go:build arduino
+//go:build arduino_uno
 
 package machine
 

src/machine/board_atmega328p.go

@@ -1,4 +1,4 @@
-//go:build (avr && atmega328p) || arduino || arduino_nano
+//go:build (avr && atmega328p) || arduino_uno || arduino_nano
 
 package machine