Move plugin help-center files to @yoast/help-center package

[LisanneKluitmans]

Summary

We want to have structured packages for all our components. To achieve this, we need to move the plugin help-center files to a new package.

Explanation

These plugin help-center files need to be moved to the new @yoast/help-center package: wordpress-seo/js/src/wp-seo-help-center.js

Technical decisions

The version of the new package needs to be 0.1.0

Feedback?