Use Serial1 without including "Adafruit_TinyUSB.h"

**Is your feature request related to a problem? Please describe.**

the following code causes `undefined reference to 'Adafruit_USBD_CDC::available()'` when targeting the nRF52840 boards.
I understand that the header "Adafruit_TinyUSB.h" needs to be included, but I don't want to include it for power saving.

```
void setup() {
Serial1.begin(115200);
}

void loop() {
}
```

This is because serialEventRun refers to Serial.
https://github.com/adafruit/Adafruit_nRF52_Arduino/blob/d75bb2a8524e7f00f725eb2ab27a8996be65e7a3/cores/nRF5/Uart.cpp#L27

How about changing the call to Serial.available to a call to the weak symbol?

```
int Serial_available() __attribute__((weak));

void serialEventRun(void)
{
if (serialEvent && Serial_available() ) serialEvent();
}
```

Start in cores/nRF5/Uart.cpp at the serialEventRun reference linked in the issue, then reproduce the sketch on an nRF52840 target without including Adafruit_TinyUSB.h. Compare the Serial1-only build with the proposed weak-symbol behavior; done means it builds without the undefined reference while preserving serialEvent handling.

Written by the indexing model from the issue text.