".js" extension is missed in main attribute in package.json

Created by: davidmtobias

https://github.com/twbs/bootstrap/blob/dca1ab7d877bc4b664b43604657a2b5fbe2b4ecb/package.json#L91

This issue makes that "require bootstrap" configuration fails. For example, using Sprockets with Rails 6, It's neccesary to add the whole path of bootstrap "require bootstrap/dist/js/bootstrap.min" in application.js instead of simply "require bootstrap"