Build/PHPCS: variable naming consistency (YoastCS exception)
Build/PHPCS: variable naming consistency (YoastCS exception)

Historically, this library uses `camelCaps` for variable names instead of the WP convention of using `snake_case`.

There is no need to change this, we just need:
* to allow for it, i.e. exclude the WPCS sniff;
* make sure it's applied consistently, i.e. include a PHPCS sniff to check for this;
* fix up the few exceptions to the rules - done in separate PRs.

Yoast/whip