Support `#include` directives in nodes

[mskelton]

Re-opening #8

I'm using this for a Kinesis advantage 360 keyboard which uses ZMK. It is possible to use #include directives inside of nodes so that you can write code like this:

/ {
  behaviors {
    #include "macros.dtsi"
  };
  keymap {
    ...
  }
}

[joelspadin]

Thanks!