Only allow passing functions to lifecycle methods like onMount.

src/runtime/internal/lifecycle.ts

@@ -11,19 +11,19 @@ export function get_current_component() {
 	return current_component;
 }
 
-export function beforeUpdate(fn) {
+export function beforeUpdate(fn: () => any) {
 	get_current_component().$$.before_update.push(fn);
 }
 
-export function onMount(fn) {
+export function onMount(fn: () => any) {
 	get_current_component().$$.on_mount.push(fn);
 }
 
-export function afterUpdate(fn) {
+export function afterUpdate(fn: () => any) {
 	get_current_component().$$.after_update.push(fn);
 }
 
-export function onDestroy(fn) {
+export function onDestroy(fn: () => any) {
 	get_current_component().$$.on_destroy.push(fn);
 }