fix: Add index.d.ts to package files.

[yeldiRium]

Description of change

Hi, another PR from me. I missed that you use the files field in the package.json file to list the files contained in the npm package, and thus did not include the index.d.ts file. It's currently missing from the npm package.

This PR adds it :)

Checklist

• Unit tests have been added
• Specific notes for documentation, if applicable

[tclindner]

Thank you!