fix(pkg): add a default fallback export

[wolfy1339]

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

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Before the change?

• Some consumers of this package could not resolve it properly (ex: jest, ts-node, tsx)
• CJS consumers would be getting errors even though the package is ESM

After the change?

• 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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