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

app/drivers/zephyr/CMakeLists.txt

@@ -5,6 +5,7 @@ if(CONFIG_ZMK_KSCAN_GPIO_DRIVER)
   zephyr_library_sources(
     kscan_gpio_matrix.c
     kscan_gpio_direct.c
+    kscan_gpio_demux.c
     )
 
   zephyr_library_sources_ifdef(CONFIG_EC11 ec11.c)