fix: use exports field for ESM exports

[wolfy1339]

See octokit/core.js#667
See octokit/core.js#665

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Before the change?

• Multiple builds for different environments
• Doesn't use ESM best practices
• CJS consumers would be getting errors even though the package is ESM

After the change?

• Only one bundle
• Uses exports field in package.json
• Clients should be able to import the module without any errors with the fallback
• CJS consumers will generate a better error with the new fallback

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes
• No

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[github-actions]

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[github-actions]

🎉 This PR is included in version 13.2.4 🎉

The release is available on:

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• npm package (@latest dist-tag)

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