Retrieve current pin mode

There is currently no function to get the current pin mode. A possible implementation is:

```
int pinMode(uint8_t pin)
{
if (pin >= NUM_DIGITAL_PINS) return (-1);

uint8_t bit = digitalPinToBitMask(pin);
uint8_t port = digitalPinToPort(pin);
volatile uint8_t *reg = portModeRegister(port);
if (*reg & bit) return (OUTPUT);

volatile uint8_t *out = portOutputRegister(port);
return ((*out & bit) ? INPUT_PULLUP : INPUT);
}
```

The return value is the pin mode or error code (-1) for illegal pin number.

Cheers!

Start with the existing pinMode API and the digitalPinToBitMask, digitalPinToPort, portModeRegister, and portOutputRegister entry points shown in the issue. Check how the hardware-independent API exposes pin operations and how cores implement them. Done means a current pin mode can be retrieved, with the specified modes returned and -1 for an invalid pin.

Written by the indexing model from the issue text.