Add `index.d.ts` with interfaces for exported classes and arguments

[tripodsan]

It would be useful to have type definitions for all exported classes.
A good example is:

• https://github.com/mozilla/source-map/blob/master/source-map.d.ts

also see:

• https://www.typescriptlang.org/docs/handbook/declaration-files/publishing.html

[trieloff]

In helix-pipeline we are generating these from the schema.

[adobe-bot]

🎉 This issue has been resolved in version 2.7.0 🎉

The release is available on:

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

Your semantic-release bot 📦🚀