libraries/Wire: Add support for target mode.

[iabdalkader]

Test sketch:

#include <Wire.h>

void onRequest() {
  Wire.write("HelloWorld!");
}

void onReceive(int howMany) {
  while (Wire.available()) {
    char c = Wire.read();
    Serial.print(c);
  }
  
  if (howMany) {
    Serial.println("");
  }
}

void setup() {
  Serial.begin(115200);
  Wire.begin(8);
  Wire.onRequest(onRequest);
  Wire.onReceive(onReceive);
}

void loop() {
  delay(1000);
}

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Retested master mode with VL53L1X and target mode with bus pirate:

[iabdalkader]

#169