CSF: Make initialGlobals change only on the external API

[shilman]

Closes N/A
Telescopes on #27517

What I did

Update #27517 to use initialGlobals for the external API, but continue to use X.globals as the internal return type from normalizeGlobals.

Also return both X.globals AND X.initialGlobals from composeConfigs for backwards compatibility.

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

1. Create a sandbox that sets initialGlobals in its .storybook/preview.js.
2. Change it to globals and verify that everything still works.

Documentation

Intentionally leaving this undocumented for now. The preview.js globals annotation was also undocumented.

🦋 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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