fix: wait a longer tick before checking slots

It seems that browser resolve "await Promise.resolve()" extra fast (close to synchronous) so the DOM isn't actually updated further. This fixes an issue where web component slots don't appear when the script registering the web components is invoked before them being added to the DOM
fixes #8997

Author: dummdidumm

.changeset/tiny-walls-brake.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: wait a longer tick before checking slots

packages/svelte/src/runtime/internal/Component.js

@@ -215,7 +215,7 @@ if (typeof HTMLElement === 'function') {
 			this.$$cn = true;
 			if (!this.$$c) {
 				// We wait one tick to let possible child slot elements be created/mounted
-				await Promise.resolve();
+				await new Promise((f) => setTimeout(f)); // don't do Promise.resolve() as that fires too soon
 				if (!this.$$cn) {
 					return;
 				}
@@ -316,7 +316,8 @@ if (typeof HTMLElement === 'function') {
 		disconnectedCallback() {
 			this.$$cn = false;
 			// In a microtask, because this could be a move within the DOM
-			Promise.resolve().then(() => {
+			// don't do Promise.resolve() as that fires too soon
+			new Promise((f) => setTimeout(f)).then(() => {
 				if (!this.$$cn) {
 					this.$$c.$destroy();
 					this.$$c = undefined;