[General] PSR-12 support

[Guite]

Since PSR-2 is deprecated consider switching to PSR-12

See also:

• https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-12-extended-coding-style-guide-meta.md#5-changelog-from-psr-2
• https://github.com/FriendsOfPHP/PHP-CS-Fixer/issues/4502
• https://github.com/karriereat/php-code-quality/pull/9

[Guite]

Reminder: put declare(strict_types=1); below the file comment block.

This includes updating StyleCI config because currently it does not like it (see aa4217c9d8a0e338000df684f2e34d05bfc051f6)

[Guite]

https://docs.styleci.io/presets#psr12