fix: prefer ts in unclear migration situations if tsconfig.json found (#12881)

Fixes #12880, relies on sveltejs/svelte#13929

Author: dummdidumm

.changeset/rare-glasses-smell.md

@@ -0,0 +1,5 @@
+---
+'svelte-migrate': patch
+---
+
+fix: prefer TS in unclear migration situations if `tsconfig.json` found

packages/migrate/migrations/svelte-5/index.js

@@ -149,6 +149,8 @@ export async function migrate() {
 
 	update_pkg_json();
 
+	const use_ts = fs.existsSync('tsconfig.json');
+
 	// const { default: config } = fs.existsSync('svelte.config.js')
 	// 	? await import(pathToFileURL(path.resolve('svelte.config.js')).href)
 	// 	: { default: {} };
@@ -170,7 +172,7 @@ export async function migrate() {
 		if (extensions.some((ext) => file.endsWith(ext))) {
 			if (svelte_extensions.some((ext) => file.endsWith(ext))) {
 				update_svelte_file(file, transform_module_code, (code) =>
-					transform_svelte_code(code, migrate, { filename: file })
+					transform_svelte_code(code, migrate, { filename: file, use_ts })
 				);
 			} else {
 				update_js_file(file, transform_module_code);

packages/migrate/migrations/svelte-5/migrate.js

@@ -56,8 +56,8 @@ export function transform_module_code(code) {
 
 /**
  * @param {string} code
- * @param {(source: string, options: { filename?: string }) => { code: string }} transform_code
- * @param {{ filename?: string }} options
+ * @param {(source: string, options: { filename?: string, use_ts?: boolean }) => { code: string }} transform_code
+ * @param {{ filename?: string, use_ts?: boolean }} options
  */
 export function transform_svelte_code(code, transform_code, options) {
 	return transform_code(code, options).code;