fix(package): include d.ts

[jimmywarting]

The purpose of this PR is:

... to add type defenition for other developers

This is what had to change:

• package.json

---

• [x] I prefixed the PR-title with docs:, fix(area):, feat(area): or breaking(area):
• [ ] I updated ./CHANGELOG.md with a link to this PR or Issue

---

• fix #126