fix: add `anchor` support to mount() API (#11050)

* fix: add `anchor` support to mount() API

* use ??

* fix inconsistent formatting

* required options before optional ones

* regenerate types

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

.changeset/afraid-geckos-dance.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: add `anchor` support to mount() API

packages/svelte/src/internal/client/render.js

@@ -90,6 +90,7 @@ export function stringify(value) {
  * @param {import('../../index.js').ComponentType<import('../../index.js').SvelteComponent<Props, Events>>} component
  * @param {{
  * 		target: Document | Element | ShadowRoot;
+ * 		anchor?: Node;
  * 		props?: Props;
  * 		events?: { [Property in keyof Events]: (e: Events[Property]) => any };
  * 		context?: Map<any, any>;
@@ -98,7 +99,7 @@ export function stringify(value) {
  * @returns {Exports}
  */
 export function mount(component, options) {
-	const anchor = options.target.appendChild(empty());
+	const anchor = options.anchor ?? options.target.appendChild(empty());
 	// Don't flush previous effects to ensure order of outer effects stays consistent
 	return flush_sync(() => _mount(component, { ...options, anchor }), false);
 }

packages/svelte/types/index.d.ts

@@ -300,6 +300,7 @@ declare module 'svelte' {
 	 * */
 	export function mount<Props extends Record<string, any>, Exports extends Record<string, any>, Events extends Record<string, any>>(component: ComponentType<SvelteComponent<Props, Events, any>>, options: {
 		target: Document | Element | ShadowRoot;
+		anchor?: Node | undefined;
 		props?: Props | undefined;
 		events?: { [Property in keyof Events]: (e: Events[Property]) => any; } | undefined;
 		context?: Map<any, any> | undefined;