feat: Add function to erase all settings

Added a zmk_settings_erase() function to clear all saved settings. This
does not go through Zephyr's settings subsystem, but instead directly
clears the data from the setting storage backend, so a reboot is needed
for it to take effect.

app/src/settings/reset_settings_fcb.c

@@ -0,0 +1,15 @@
+/*
+ * Copyright (c) 2023 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <zmk/settings.h>
+
+#include <zephyr/logging/log.h>
+LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
+
+int zmk_settings_erase(void) {
+    LOG_ERR("Settings reset is not implemented for deprecated FCB backend.");
+    return -ENOSYS;
+}