Changelog for WordPressCS 3.0.0 (#2368)

This changelog is based on going through every single PR merged in the WordPressCS 3.0.0 milestone.

The changelog includes only limited, high-level information about the internal changes. More detailed information about the changes to the internals have been included in the upgrade guide for developers/extenders.
This means that things like removing a method from a class which is now `final` has not been listed in the changelog (as the `final` keyword prevents extension anyway, so the method removal is not relevant for extenders).

In contrast to earlier changelogs, I have chosen to credit people who have contributed to the release with "Props" mentions.
Note: the "Props" have only been added for contributions from people outside of the WPCS team.

Author: jrfnl