feat: :label: Added `title` attribute in the interface `Step`

[adevinwild]

Ref to : https://github.com/HiDeoo/intro.js-react/issues/89

Describe the pull request

Added the title attribute inside the Step interface, it works without errors.

Why

The title attribute was missing, and can be used without any errors.

How

By adding a new key inside the interface Step inside the index.d.ts file.

Screenshots