Add PHPCompatibility rules for PHP 7.2

[rmccue]

We should include the rules from PHPCompatibility for PHP 7.2 in the base set of rules, to ensure all greenfields code is compatible for when we roll it out to servers.

[rmccue]

Specifically, the WP rules.

[rmccue]

@tfrommen Do you have any specific code for this you can PR? 🙏

[tfrommen]

Now I do, see #81. 🙂