function onDestroy(fn) {
	if (component_context === null) {
		lifecycle_outside_component('onDestroy');
	}

	onMount(() => () => untrack(fn));
}
Schedules a callback to run immediately before the component is unmounted.

Out of `onMount`, `beforeUpdate`, `afterUpdate` and `onDestroy`, this is the
only one that runs inside a server-side component.

@param {() => any} fn
@returns {void}