Bug: digitalWrite() incorrectly uses DIRSET register instead of DIR register for pin direction checking

## Bug Description
The current implementation of `digitalWrite()` in the SAMD core incorrectly uses the DIRSET register to determine if a pin is configured as an output, which could lead to inconsistent behavior.

## Technical Details
In `wiring_digital.c`, the function uses the following condition to check if a pin is configured as an output:

```c
if ((PORT->Group[port].DIRSET.reg & pinMask) == 0)
{
// the pin is not an output, disable pull-up if val is LOW, otherwise enable pull-up
PORT->Group[port].PINCFG[pin].bit.PULLEN = ((ulVal == LOW) ? 0 : 1);
}
```

However, according to the SAMD21 datasheet (section 23.8.3), DIRSET is designed to be a write-only register and does not guarantee it matches actual pin direction state. This will cause the function to sometimes incorrectly attempt configuring pull-up resistors even on pins configured as outputs.

## Proposed Fix

Replace the DIRSET check with a DIR check, which correctly reflects the current pin direction state:
```c
if ((PORT->Group[port].DIR.reg & pinMask) == 0)
{
// [rest of the code unchanged]
}
```

## Impact Assessment
This bug has likely been present for some time without causing major functional issues, because

1. setting pull-up configuration on output pins has minimal effect (output drivers override pull-ups).
2. modifying `DIRCLR` and `DIRTGL` registers updates `DIRSET` so it aligns with `DIR`.
3. Testing shows `DIRSET` tracks `DIR` even under circumstances not guaranteed by the datasheet

While not a high impact issue, it's technically incorrect and could potentially cause subtle issues in certain edge cases or confuse developers examining the behavior of the function.

Open wiring_digital.c and inspect digitalWrite(), then read SAMD21 datasheet section 23.8.3 to compare DIRSET with DIR. The change is complete when the output-direction check uses the register that reflects current pin direction and the existing behavior remains unchanged for pull-up handling.

Written by the indexing model from the issue text.