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.

首先查看 Stream/HardwareSerial 以及 issue 中链接的早期 Arduino PRs，以了解提议的 API 和受影响的 core 实现。确认当前如何公开传输缓冲区状态，然后将完成标准定义为：Serial.isFlushed() 能可靠地报告所有传输数据都已离开缓冲区，以供 half-duplex 使用，并在相关 core 测试串行行为的地方提供相应的覆盖率。

由索引模型根据 Issue 内容生成。