fix: correct type export to use .d.ts extension

[kachick]

Since v15 era, looks like related to #909
And for my case, this is not yet fixed in v15.4.1(#923)

I'm not an expert for this domain, but in my understanding, omitting extensions for .d.ts expect the related .js. Currently there is no schema.js in the package. microsoft/TypeScript#48508 (comment)

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Before the change?

• Cannot use the exported types, at least for CheckSuite, Workflow, CheckRun, Commit
• my use-case: https://github.com/kachick/wait-other-jobs/blob/2f486e6edf4772ae8aa7f4776c8ba6b079aef33e/src/github-api.ts#L3
• 
• https://github.com/arethetypeswrong/arethetypeswrong.github.io display errors for ESM packaging
  

After the change?

• 
• 

Pull request checklist

• Tests for the changes have been added (for bug fixes / features) => Added in chore: add test for exported types in npm package #925
• 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 15.4.2 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀

[kachick]

Thank you!