Make `HardwareSerial::write(buf, len)` non-blocking

`Serial1.write(buf, len)` (`HardwareSerial`) is blocking on Arduino Uno R4 Minima.

I kindly propose as a feature request that it be made non-blocking.

Here is the used sketch:

```
uint8_t Tx[] = {0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09};

void setup() {
pinMode(4, OUTPUT);
Serial1.begin(19200);
}

void loop() {

// send 10B
Serial1.write(Tx, 10);

// wait while toggling pin
uint32_t tStart = millis();
while (millis() - tStart < 200)
digitalWrite(4, !digitalRead(4));
}

```

and the measurement of the test pin:

### Additional context

`Serial.write()` is non-blocking even on the old 8-bit AVRs w/o DMA.

Start at the HardwareSerial::write(buf, len) implementation used by Serial1 on the Arduino Uno R4 Minima and compare its behavior with the non-blocking Serial.write() behavior on AVR boards. Use the provided sketch and test-pin measurement to reproduce the blocking, then verify that the write call no longer blocks while preserving the requested transmission.

Written by the indexing model from the issue text.