Support PHP_CodeSniffer 4.0

[takaram]

PHP_CodeSniffer 4.0.0beta1 has been released.
Currently, the composer.json of slevomat/coding-standard does not allow 4.0.0:

coding-standard/composer.json

Line 19 in 1501a61

"squizlabs/php_codesniffer": "^3.12.2"

Is it possible to update the version constraint to allow "^4.0"?

Thank you for your work on this great coding standard! 🙏

[pieterocp]

There will end up needing to be a bit more than just a version constraint change, e.g. see the branch phpcs4 + will likely require a few more changes.