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_none.c

@@ -0,0 +1,12 @@
+/*
+ * Copyright (c) 2023 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <zmk/settings.h>
+
+int zmk_settings_erase(void) {
+    // No storage backend; nothing to do
+    return 0;
+}