fix: ISO's types with TS's "NodeNext" resolution

Author: rschristian

packages/preact-iso/index.d.ts

@@ -1,4 +1,4 @@
-export { default as prerender } from './prerender';
-export * from './router';
-export { default as lazy, ErrorBoundary } from './lazy';
-export { default as hydrate } from './hydrate';
+export { default as prerender } from './prerender.js';
+export * from './router.js';
+export { default as lazy, ErrorBoundary } from './lazy.js';
+export { default as hydrate } from './hydrate.js';