CLI: Improve how automigrations read main.js

[yannbf]

Closes #20955

What I did

Followup work from #20647 regarding #20955.
This PR refactors the remaining automigrations to use the improved helpers created in #20647 in order to read main.js by evaluating it, instead of parsing its AST, making it possible for any automigration to detect its case in non-standard Storybook configurations, while possibly failing at the write level (if the user runs the migration).

How to test

1. Access a migratable Storybook, e.g. https://github.com/yannbf/storybook-coverage-recipes
2. Bootstrap dependencies in the monorepo
3. Run path/to/repo/lib/cli/index.js automigrate in that project
4. Things should work correctly

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"]

[shilman]

Great work @yannbf 💯