Add an example to read VBUS and VSYS (#331)

The process is different on Pico and Pico W so demonstrate how to do it.

Fixes #324

adc/read_vsys/CMakeLists.txt

@@ -0,0 +1,30 @@
+add_library(power_status_adc INTERFACE)
+target_sources(power_status_adc INTERFACE
+        ${CMAKE_CURRENT_LIST_DIR}/power_status.c
+        )
+target_include_directories(power_status_adc INTERFACE
+        ${CMAKE_CURRENT_LIST_DIR}
+        )
+target_link_libraries(power_status_adc INTERFACE
+        hardware_adc
+        hardware_gpio
+        )
+
+add_executable(read_vsys
+    read_vsys.c
+    )
+target_include_directories(read_vsys PRIVATE
+    ${CMAKE_CURRENT_LIST_DIR}
+    )
+target_link_libraries(read_vsys
+    pico_stdlib
+    power_status_adc
+    )
+if (PICO_CYW43_SUPPORTED)
+    target_link_libraries(read_vsys
+        pico_cyw43_arch_none
+    )
+endif()
+
+pico_add_extra_outputs(read_vsys)
+example_auto_set_url(read_vsys)