Add persistent USB output mode selection

tests/mode_native_stubs/pico/time.h

@@ -0,0 +1,12 @@
+#pragma once
+
+#include <stdint.h>
+
+typedef int32_t alarm_id_t;
+typedef uint64_t absolute_time_t;
+typedef int64_t (*alarm_callback_t)(alarm_id_t alarm_id, void* user_data);
+
+absolute_time_t get_absolute_time();
+uint64_t to_ms_since_boot(absolute_time_t time);
+alarm_id_t add_alarm_in_ms(int64_t delay_ms, alarm_callback_t callback,
+                           void* user_data, bool fire_if_past);