Add wp-compat PHPStan extension

[szepeviktor]

Summary

Fixes #1535

@swissspidy is my favorite troublemaker 🙃 so I do what he says.

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[swissspidy]

johnbillion/wp-compat 0.2.1 requires php >= 7.4 -> your php version (7.2; overridden via config.platform, actual: 8.3.11) does not satisfy that requirement.

This is why we can't have nice things 😞

[westonruter]

The minimum PHP version was bumped to 7.2 in 6.6. It seems there is some momentum to get on newer versions. So perhaps 7.4 will follow soon?

[szepeviktor]

I usually say that the WordPress ecosystem (core+3rd-party themes/plugins) is PHP 7.4 compatible.
https://wordpress.org/about/stats/#php_versions