CI Github actions setup: psalm and php-cs-fixer

[bopoda]

1. Set up GitHub Actions CI:

• Run phpunit on PHP 7.4, PHP 8.0, PHP 8.1, PHP 8.2, PHP 8.3, PHP 8.4. Support since PHP 7.4.
• Run psalm (PHP 8.3)
• Run php-cs-fixer (PHP 8.3). Fixer is applied to the folders src, tests of the codebase.

2. Minor code changes.