Set specific coding standard and improve code quality

[Sebbo94BY]

• Configures PHP-CS-Fixer rules for a specific coding standard in order to improve the code quality
• Applies the PHP-CS-Fixer rules to the current existing code
• Adds a Github action workflow to ensure, that this coding standard always gets applied

$ ./vendor/bin/php-cs-fixer fix --config .php-cs-fixer.php --diff --dry-run
Loaded config default from ".php-cs-fixer.php".
Using cache file ".php-cs-fixer.cache".

Found 0 of 94 files that can be fixed in 0.015 seconds, 12.000 MB memory used