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/include/zmk/settings.h

@@ -0,0 +1,15 @@
+/*
+ * Copyright (c) 2023 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+
+/**
+ * Erases all saved settings.
+ *
+ * @note This does not automatically update any code using Zephyr's settings
+ * subsystem. This should typically be followed by a call to sys_reboot().
+ */
+int zmk_settings_erase(void);