Handle dynamically adding items in list components: Radio Group, Tabs, Listbox, Menu

Fixes #29

Upstream Headless UI handles this for Radio Group but not for the others. I didn't see any reason why not to implement this for the other ones too.

src/lib/components/radio-group/radio-group.test.ts

@@ -124,8 +124,7 @@ describe('Rendering', () => {
     assertNotFocusable(getByText('Dine in'))
   })
 
-  // TODO: fix this test!
-  it.skip('should guarantee the radio option order after a few unmounts', async () => {
+  it('should guarantee the radio option order after a few unmounts', async () => {
     render(svelte`
       <script>
         let showFirst = false;