Add libgpiod v2 support and fix Pi 5 compatibility

[momenso]

The existing libgpiod path only supported the v1 API, which is incompatible with Raspberry Pi OS on Debian 12+ where libgpiod v2 is the default. This PR adds full v2 support alongside the existing v1 path, with the version selected automatically at build time.

• We now use pkg-config to detect the installed libgpiod major version at configure time and sets GPIOD_V1 or GPIOD_V2 accordingly. bcm2835.c is excluded from the build when --use_libgpiod=true is passed.

• The v2 path uses the new gpiod_chip_request_lines API with OPEN_DRAIN output mode, which is required for the single-wire DHT protocol on the RP1 southbridge. GPIO chip discovery now enumerates available chips by their kernel label (pinctrl-rp1) , with a device-path fallback for forward compatibility. The v1 path is updated to release and re-request lines on direction change for compatibility with libgpiod < 1.5.

• Installation section restructured to clearly separate the Pi 5 / libgpiod path from the default BCM2835 path. Node.js installation appendix updated to use NodeSource.

Closes #149.