Feature request: Add INPUT_PULLDOWN support to pinPeripheral()

[sslupsky]

The pinPeripheral() function in wiring_private.c has support for "PIO_INPUT", "PIO_INPUT_PULLUP" and "PIO_OUTPUT". It lacks support for the "PULL DOWN" configuration.

Interestingly, pinMode() does support "INPUT_PULLDOWN".

I propose that we add:

      if ( ulPeripheral == PIO_INPUT_PULLDOWN )
      {
        pinMode( ulPin, INPUT_PULLDOWN ) ;
      }

We will also need to add PIO_INPUT_PULLDOWN to the EPioType in WVariant.h.

I would be happy to submit a PR for this. Before I do, I have one question regarding suggestions of how best to set up my environment to use the ArduinoCore-samd Git repo instead of the core that is installed with the IDE.

[sslupsky]

BTW, I have a few other PR's to suggest but the environment thing is preventing me from doing so. There doesn't seem to be a preference in the IDE that allows me to add or change the directory of the cores or tools.