Change default coding standard from PSR-2 to PSR-12

[krzysztof-sikorski]

On 2019-08-10 the PSR-2 coding standard has been deprecated in favor of PSR-12.

I think this package should be updated accordingly.

Sources:

• https://www.php-fig.org/psr/psr-2/
• https://www.php-fig.org/psr/psr-12/
• https://www.php-fig.org/blog/2019/09/new-blog-psr-12-approval/#psr-12-has-been%C2%A0approved