add missing export

Author: nathancahill

src/index.js

@@ -2,3 +2,4 @@ export { default as Sveltik } from './Sveltik.svelte'
 export { default as Form } from './Form.svelte'
 export { default as Field } from './Field.svelte'
 export { default as ErrorMessage } from './ErrorMessage.svelte'
+export { default as ScrollMarker } from './ScrollMarker.svelte'