YoastCS: drop support PHP < 7.2 & modernize the codebase

[jrfnl]

Drop support for PHP < 7.2

The majority of the Yoast codebases now have a minimum of PHP 7.2.

Additionally, the YoastCS checks should give the same results independently of the PHP version on which the CS check is being run, which means that generally speaking, the CS check is run on a high PHP version, both locally as well as in CI.

With this in mind, there is no reason to keep the YoastCS minimum supported PHP version at PHP 5.4, so let's bring it in line with the other Yoast codebases and let it have a PHP 7.2 minimum.

Note: PHP_CodeSniffer itself still has a PHP 5.4 minimum. PHPCS 4.x is expected to have a PHP 7.2 minimum and this change is in line with that.

YoastCS/Modernize: add explicit visibility to all OO constants

YoastCS/Modernize: use constant arrays

Prior to PHP 5.6 class constants couldn't contain arrays. As support for PHP < 7.2 has now been dropped, unchangable properties containing arrays should be changed to constants.

YoastCS/Modernize: selectively use null coalescing

PHP 7.0 introduced the null coalesce operator. This commit implements the use of the operator in select places.

YoastCS/Modernize - Tests: add PHP native return type declarations

As support for PHP < 7.2 has now been dropped, PHP native parameter and return type declarations can be used.

There are, however, a couple of caveats:

1. Return type declarations can be freely used as long as the type needed is available in PHP 7.2.
2. Parameter type declarations can not be freely used as parameter types are contravariant, which means they can only be wider than the original type, which means parameter type declarations can only be added for YoastCS native methods and not for methods which implement a PHPCS native interface method or overload a PHPCS native sniff method, as in that case, the original method will rarely have a type declaration, which means that adding one in YoastCS would violate LSP.

Also note that a TypeError thrown by a sniff, would stop the scan of a file dead, so let's be conservative with adding type declarations in the sniffs for now, but add them freely to the tests.

[coveralls]

coverage: 97.146% (-0.5%) from 97.644%
when pulling c4f44a9 on JRF/yoastcs-drop-support-php-lt-7.2
into 1acd027 on develop.