fix(ComposedModal): remove bx--body--with-modal-open from body classes after destroy

When onMount is async, the returned function is not executed: https://github.com/sveltejs/svelte/issues/4927
Because the class is not remove from the body, the code should be moved to a onDestroy function.

src/ComposedModal/ComposedModal.svelte

@@ -47,6 +47,7 @@
     setContext,
     onMount,
     afterUpdate,
+    onDestroy,
   } from "svelte";
 
   const dispatch = createEventDispatcher();
@@ -79,12 +80,12 @@
   onMount(async () => {
     await tick();
     focus();
-
-    return () => {
-      document.body.classList.remove("bx--body--with-modal-open");
-    };
   });
 
+  onDestroy(() => {
+    document.body.classList.remove("bx--body--with-modal-open");
+  })
+
   afterUpdate(() => {
     if (opened) {
       if (!open) {