Neopixel fix

[SuGlider]

Description of Change

Based on issue #8841, it was clear that some improvements could be done to the neopixelWrite() function.
Changes:
1- All RMT examples use the PIN_NEOPIXEL defined in pins_arduino.h for the Espressif devkits
2- neopixelWrite() can be used with many GPIOs in the same sketch. Before this changes, the sketch could only use one GPIO with neopixelWrite().

Tests scenarios

Tested with ESP32-C6:

void blinkRGBLED(uint32_t count, uint32_t time_ms) {
  const uint8_t LED_Pin = 23;  // change it to any other valid GPIO
  for ( ; count; count--) {
    neopixelWrite(LED_Pin, 255, 255, 255);  // RGB = White (on)
    Serial.println("HIGH");
    delay(time_ms);
    neopixelWrite(LED_Pin, 0, 0, 0); // RGB = Black (off)
    Serial.println("LOW");
    delay(time_ms);
  }
}

void setup() {
  // No need to initialize the RGB LED
  blinkRGBLED(5, 500);
}

// the loop function runs over and over again forever
void loop() {
#ifdef RGB_BUILTIN
  digitalWrite(RGB_BUILTIN, HIGH);   // Turn the RGB LED white
  delay(1000);
  digitalWrite(RGB_BUILTIN, LOW);    // Turn the RGB LED off
  delay(1000);

  neopixelWrite(PIN_NEOPIXEL, RGB_BRIGHTNESS, 0, 0); // Red
  delay(1000);
  neopixelWrite(PIN_NEOPIXEL, 0, RGB_BRIGHTNESS, 0); // Green
  delay(1000);
  neopixelWrite(PIN_NEOPIXEL, 0, 0, RGB_BRIGHTNESS); // Blue
  delay(1000);
  neopixelWrite(PIN_NEOPIXEL, 0, 0, 0); // Off / black
  delay(1000);
#endif
}

Related links

Fix #8841