add isFlushed() to Stream/HardwareSerial

Please add Serial.isFlushed() method for checking that all data in Tx buffer have been flushed. We need it for non-blocking Serial.write() to half duplex communication (such as RS485).
At the moment I use something like this in my non-blocking code:

```
while (Serial.availableForWrite() > 0 && ndx < sizeof(buffer)) {
Serial.write(buffer[ndx]);
ndx++;
}
if (Serial.availableForWrite() == SERIAL_TX_BUFFER_SIZE - 1 && ndx > sizeof(buffer)) {
ndx = 0;
// Disable RS485 transmit and start receiving
}
```

But Serial.availableForWrite() == SERIAL_TX_BUFFER_SIZE - 1 is not reliable for checking that Tx buffer has been flushed.

Here is an earlier PR which was never merged:
https://github.com/arduino/Arduino/pull/3737

and a similar PR for ArduinoCore-sam here:
https://github.com/arduino/ArduinoCore-sam/pull/7

I would help, but coding Arduino core is beyond my skills.

Start by reviewing Stream/HardwareSerial and the earlier Arduino PRs linked in the issue to understand the proposed API and affected core implementations. Verify how transmit-buffer state is exposed today, then define done as Serial.isFlushed() reliably reporting that all transmit data has left the buffer for half-duplex use, with matching coverage where the relevant cores test serial behavior.

Written by the indexing model from the issue text.