Vue3 no longer exports app in 7.0-alpha

[shilman]

This commit removes the app export from @storybook/vue3:

24251e2#diff-63c4fc9feba520d3505ec8a6796e7d0576ef804b6f92d68ae8b8c2e64f18264c

We should try to restore it so that users can use it to add global components and directives.

[matsko]

I've also run into this issue.

[matsko]

Looks like this is the current approach:

import {setup} from "@storybook/vue3";

setup((app => {
  return setupMyApp(app);
}));