Use PHPStan to improve related-posts-by-taxonomy

[Viktor Szépe]

Here is the config: phpstan.neon.dist

includes:
    - autoloader/szepeviktor/phpstan-wordpress/extension.neon
parameters:
    level: 5
    inferPrivatePropertyTypeFromConstructor: true
    paths:
        - includes/
    excludes_analyse:
        - includes/assets/
    ignoreErrors:
        # Uses func_get_args()
        - '#^Function apply_filters(_ref_array)? invoked with [34567] parameters, 2 required\.$#'