feat: allow vue files in importfn

[tobiasdiez]

What I did

If one uses https://github.com/tobiasdiez/storybook-vue-addon/ (which enables stories in native vue format), then one get warnings of the form

Cannot process .vue file with storyStoreV7: src/components/Button.stories.vue

This is fixed by allowing vue files.

How to test

I've noticed this issue in the context of tobiasdiez/storybook-vue-addon#38 (run pnpm example:vite)

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• If this PR should be tested against many or all sandboxes,
  make sure to add the ci:merged or ci:daily GH label to it.
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "documentation", "maintenance", "dependencies", "other"]

[JReinhold]

This is definitely not a scalable API, but until we come up with something better, it's what we can do.

For anyone else, see related #20442