Replace popper.js with @popperjs/core

[nelsonni]

Per the instructions on the popper.js package:

This package has been deprecated

Author message:
You can find the new Popper v2 at @popperjs/core, this package is dedicated to the legacy v1

Therefore, @popperjs/core needs to replace popper.js in the package.json file. Any bugs or issues that appear due to the newer API should also be resolved prior to accepting the update PR.