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_file.c

@@ -0,0 +1,23 @@
+/*
+ * Copyright (c) 2023 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <zephyr/fs/fs.h>
+
+#include <zmk/settings.h>
+
+#include <zephyr/logging/log.h>
+LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
+
+int zmk_settings_erase(void) {
+    LOG_INF("Erasing settings file");
+
+    int rc = fs_unlink(CONFIG_SETTINGS_FS_FILE);
+    if (rc) {
+        LOG_ERR("Failed to unlink '%s': %d", CONFIG_SETTINGS_FS_FILE, rc);
+    }
+
+    return rc;
+}