Add new kscan_gpio_demux driver

+ zmk,kscan_gpio_demux is now a compatible kscan option
+ kscan_gpio_demux is based heavily off of the implementation of
kscan_gpio_matrix, modified to address cols using an n-to-1
demultiplexer chip instead of discrete IO
+ Added zmk,kscan_gpio_demux.yaml file
* modified CMakeLists to include new source files

.vscode/settings.json

@@ -2,5 +2,7 @@
   "files.associations": {
     "*.overlay": "dts",
     "*.keymap": "dts"
-  }
+  },
+  "C_Cpp.dimInactiveRegions": false,
+  "C_Cpp.errorSquiggles": "Disabled"
 }