Switch to WPCS 3.0

[Slava Abakumov]

A new major version of WPCS was released, with breaking changes.

Let's start using it soon-ish.

https://github.com/WordPress/WordPress-Coding-Standards/releases/tag/3.0.0

Here is how Gutenberg is doing the switch (not merged yet): https://github.com/WordPress/gutenberg/pull/53866/files