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/examples/pininterrupt/arduino-uno.go

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

src/examples/pwm/arduino-uno.go

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

src/examples/pwm/pico2.go

@@ -0,0 +1,11 @@
+//go:build pico2 || pico_plus2
+
+package main
+
+import "machine"
+
+var (
+	pwm = machine.PWM0 // Pin 25 (LED on pico2) corresponds to PWM0.
+	//pinA = machine.LED
+	pinA = machine.GPIO16
+)