ws2812: add PIO support for RP2040/RP2350

Integrate PIO support directly into the existing Device.
NewWS2812() now uses PIO for hardware-timed control
on RP2040/RP2350 and falls back to bit-banging on other platforms.

No changes to the exported API surface.

ws2812/ws2812_init_other.go

@@ -0,0 +1,10 @@
+//go:build !rp2040 && !rp2350
+
+package ws2812
+
+import "machine"
+
+// newWS2812Device creates a WS2812 device using the bit-bang driver.
+func newWS2812Device(pin machine.Pin) Device {
+	return Device{Pin: pin, writeColorFunc: writeColorsRGB}
+}