Request adding Serial.writeIdle()

Request adding Serial.writeIdle() command;
returns 1 when no data remain in Serial TX buffer and the UART TX is idle,
returns 0 if not all serial data have been completely transmitted.

I don't see any function like this in the Serial reference ( https://www.arduino.cc/reference/en/language/functions/communication/serial/ ).  

My goal for this function is to provide a non-blocking means to know when all the data I have sent to the TX UART have been completely transmitted by the TX UART, so that I can safely call Serial.end() and avoid prematurely stopping serial transmission.  I'm using the UART TX output to control a low power RF transmitter (transmitter is active on HIGH control input) and want a way to know when I can safely turn off the UART TX output (and thus ensure that the transmitter is off).

Start by locating the Serial implementation in ArduinoCore-avr and compare its transmission behavior with the linked Serial reference. Verify that the requested command is non-blocking, reports both an empty TX buffer and an idle UART, and supports safely ending transmission.

Written by the indexing model from the issue text.