CLI: Fix eslint configuration for string extends

[shilman]

Closes N/A

What I did

Looks like ESlint config .extends can also be a string and our automigration fails in that case:

Thanks @ShaunEvening for the bug report! 🙌

Checklist for Contributors

Testing

The changes in this PR are covered in the following automated tests:

• stories
• unit tests
• integration tests
• end-to-end tests

Manual testing

Install SB in a project with a string value for eslint extends

🦋 Canary release

This PR does not have a canary release associated. You can request a canary release of this pull request by mentioning the @storybookjs/core team here.

core team members can create a canary release here or locally with gh workflow run --repo storybookjs/storybook canary-release-pr.yml --field pr=<PR_NUMBER>

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[shilman]

Self-merging @yannbf @ShaunEvening