CLI: Add main.js docs.autodocs automigration

[shilman]

Closes #27078

What I did

Instead of checking the user's main.js file at runtime to log a deprecation message, just add an automigration that runs at upgrade time.

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

Run storybook automigrate autodocs-tags in a storybook with:

• docs.autodocs = true
• docs.autodocs = true and no preview.js file
• docs.autodocs = false
• docs.autodocs = 'tag'

Verify that the migration makes sense and that the messages are useful.

Documentation

N/A

Checklist for Maintainers

• When this PR is ready for testing, make sure to add ci:normal, ci:merged or ci:daily GH label to it to run a specific set of sandboxes. The particular set of sandboxes can be found in code/lib/cli/src/sandbox-templates.ts

• Make sure this PR contains one of the labels below:

  Available labels

  • bug: Internal changes that fixes incorrect behavior.
  • maintenance: User-facing maintenance tasks.
  • dependencies: Upgrading (sometimes downgrading) dependencies.
  • build: Internal-facing build tooling & test updates. Will not show up in release changelog.
  • cleanup: Minor cleanup style change. Will not show up in release changelog.
  • documentation: Documentation only changes. Will not show up in release changelog.
  • feature request: Introducing a new feature.
  • BREAKING CHANGE: Changes that break compatibility in some way with current major version.
  • other: Changes that don't fit in the above categories.

🦋 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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[yannbf]

Some cleanup is needed:

src/modules/store/csf/prepareStory.ts(2,1): error TS6133: 'dedent' is declared but its value is never read.
src/modules/store/csf/prepareStory.ts(26,1): error TS6133: 'once' is declared but its value is never read.

[yannbf]

Looks fine to me, did not test