Provide composer.json suggesting required libraries

As noted in other issues, the 8.x-2.x branch has a couple widgets that require external JavaScript Libraries. They can be added via composer.

However, unless the main project file is using Asset Packagist, they won't be found and hence the whole project will not be installable via composer in some case. As it is only needed for a couple widgets, that doesn't make sense. So we will add it under suggests. Probably the best middle ground choice.