Refresh wsuwp-dev.sl state file

Would like to:

* [x] Clean up documentation throughout to make things more obvious.
* [x] Remove unnecessary `require` and `require_in` statements
* [x] Remove the xdebug file management to be introduced in another issue.
* [x] Remove the reliance on `wp-db-import`
* [x] Use `wp` to setup the initial development environment if not yet available.
* [x] Use `wp` to add default users for role testing:
    * `wsuwp.admin` as an administrator / super admin
    * `wsuwp.editor` as an editor
    * `wsuwp.contributor` as a contributor
    * `wsuwp.author` as an author
    * `wsuwp.subscriber` as a subscriber