fix: don't import useAsyncStoryblok twice

[yooouuri]

The composables folder is already auto imported:

addImportsDir(resolver.resolve('./runtime/composables'));

fixes #331

[maoberlehner]

Explicitly adding the import for useAsyncStoryblok was the fix for: #264
See: #294

Maybe you can add a check if auto imports are disabled?

Thank you for your contribution!