fix(types): allow mix of strings and obj in writeEarlyHintsLinks

[jonaskuske]

Checklist

• [x] run npm run test and npm run benchmark
• [x] tests and/or benchmarks are included
• [ ] documentation is changed or added
• [x] commit message and code follows the Developer's Certification of Origin and the Code of conduct

[jonaskuske]

Thanks for opening a PR! Can you please add a unit test? we use tsd for TS.

Done 👍 Passes (and I verified that it fails without the change introduced here)