Add flushReceiveBuffer() to HardwareSerial

In Serial communication sometimes one wants to flush all data in the receive buffer at once. E.g. because data has become invalid. - https://forum.arduino.cc/index.php?topic=530650 -

The code to do this very fast in HardwareSerial is straightforward,
```
void HardwareSerial::flushReceiveBuffer()
{
_rx_buffer_head = _rx_buffer_tail;
}
```

Start by locating the HardwareSerial declaration and its existing flush-related implementation or tests. Check how the receive buffer is represented and how public API additions are tested; done means the API exposes flushReceiveBuffer() and calling it discards all currently buffered receive data.

Written by the indexing model from the issue text.