fix(nuxt): do not render app pages alongside stories (fixes #472)

[medfreeman]

Description

Fixes #472

This PR stops @histoire/plugin-nuxt from rendering whatever is in nuxt app template under each story and the whole histoire page itself.
It does so by injecting an empty compiled vue template instead of the app template.
It also disables pages in nuxt.config for the plugin not to try rendering the app routes, which avoids vue-router errors in console and the rendering of 404s in place of the extra app template.

Additional context

Please tell me if i should write a test that checks that nuxt app template / 404s don't render alongside stories.
Perhaps by checking there's no additional content instead of looking for specific content.

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• Bug fix
• New Feature
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[Akryum]

Thanks! Maybe a test would be nice so we don't get unexpected regression with Nuxt in the future 👍

[medfreeman]

I've added a test, which passes w/ the fix and doesn't w/o, on both actual and latest nuxt versions.

Side question: is it wanted that the nuxt app both renders in each story and on the whole histoire page additionally?
Does and shall setupCode run for both cases?

[Akryum]

Yes because in some cases (e.g. grids) there are no iframes.