fix: V3 has white LED on GPIO35, not WS2812B on GPIO38

The Heltec V3 has a plain white LED on GPIO35 (simple GPIO, not
addressable). Only the V4 has a WS2812B RGB LED on GPIO38.

- Created driver/simple_led.rs for single-color GPIO LEDs
- V3/V3-UART boards: SimpleLed on GPIO35 (white blink on RX/TX)
- V4 board: unchanged (WS2812 on GPIO38, green/red colors)
- nRF boards: unchanged (no LED, type LedDriver = ())

src/driver/mod.rs

@@ -1,5 +1,8 @@
 #[cfg(feature = "wio_tracker_l1")]
 pub mod sh1106;
 
-#[cfg(any(feature = "heltec_v3", feature = "heltec_v3_uart", feature = "heltec_v4"))]
+#[cfg(any(feature = "heltec_v3", feature = "heltec_v3_uart"))]
+pub mod simple_led;
+
+#[cfg(feature = "heltec_v4")]
 pub mod ws2812;