refactor(composed-modal): replace onDestroy with onMount return function

src/ComposedModal/ComposedModal.svelte

@@ -36,7 +36,6 @@
     setContext,
     onMount,
     afterUpdate,
-    onDestroy,
   } from "svelte";
   import { writable } from "svelte/store";
 
@@ -73,13 +72,14 @@
   let opened = false;
   $: didOpen = open;
 
-  onMount(async () => {
-    await tick();
+  onMount(() => {
+    tick().then(() => {
       focus();
     });
 
-  onDestroy(() => {
+    return () => {
       document.body.classList.remove("bx--body--with-modal-open");
+    };
   });
 
   afterUpdate(() => {