function bind_prop(props, prop, value) {
	var desc = get_descriptor(props, prop);

	if (desc && desc.set) {
		props[prop] = value;
		teardown(() => {
			props[prop] = null;
		});
	}
}
Makes an `export`ed (non-prop) variable available on the `$$props` object
so that consumers can do `bind:x` on the component.
@template V
@param {Record<string, unknown>} props
@param {string} prop
@param {V} value
@returns {void}