This commit Adds the ability for the user to register a callback for Serial/SerialUSB events (TX/RX)

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Summary

This PR fixes/implements the following bugs/features

• Add Serial events callback

Explain the motivation for making this change. What existing problem does the pull request solve?

#include <Arduino.h>

#define SERIAL_BAUDRATE 115200U

static uint32_t count = 0;

static void onSerialEvent(SerialEvent_t event);

void setup() {
  Serial.begin(SERIAL_BAUDRATE);
  Serial.onEvent(onSerialEvent);
}

void loop() {
  Serial.print("Running(");
  Serial.print(count);
  Serial.println(")...");
  count++;
  delay(1000);
}

static void onSerialEvent(SerialEvent_t event) {
  switch (event) {
    case SERIAL_EVENT_RX:
      __NOP();
      break;
    case SERIAL_EVENT_TX:
      __NOP();
      break;
    default:
      __NOP();
      break;
  }
}

Validation

• Ensure CI build is passed.
• Demonstrate the code is solid. [e.g. Provide a sketch]

Code formatting

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Closing issues

Fixes #xxx