fix: skip Program comment nodes in migration to prevent crash (#18656)

Fixes #18304

The parent of a declaration could be the Program, on which HTML comments are added, which subsequently crashes the funciton. Ignore such comments.

---------

Co-authored-by: Simon Holthausen <simon.holthausen@vercel.com>

.changeset/fix-migrate-comment-no-position.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ignore comments of Program node during migration script

packages/svelte/src/compiler/migrate/index.js

@@ -1590,7 +1590,12 @@ function migrate_slot_usage(node, path, state) {
  */
 function extract_type_and_comment(declarator, state, path) {
 	const str = state.str;
-	const parent = path.at(-1);
+	let parent = path.at(-1);
+
+	if (parent?.type === 'Program') {
+		// We don't want comments from the program node
+		parent = undefined;
+	}
 
 	// Try to find jsdoc above the declaration
 	let comment_node = /** @type {Node} */ (parent)?.leadingComments?.at(-1);

packages/svelte/tests/migrate/samples/props-export-alias/input.svelte

@@ -1,3 +1,4 @@
+<!-- @component x -->
 <script lang="ts">
 	let klass = '';
 	export { klass as class }

packages/svelte/tests/migrate/samples/props-export-alias/output.svelte

@@ -1,3 +1,4 @@
+<!-- @component x -->
 <script lang="ts">
 	interface Props {
 		class?: string;