Introduction of .editorfconfig and php-cs-fixer.

[EvilKarter]

This is a follow-up for #10 and should be merged afterwards

This PR is also switching to PHP 8.1 like #9.
This PR introduces editrofconfig and php-cs-fixer. It serves as a basis for enforcing standardized formatting.
the Github workflow has been extended by the php-cs-fixer check and phpstan.

The current .editorfconfig was taken from the Symfony Project.
The PHP-CS-Fixer config was based on the one from Symfony. However, some rules were adjusted manually to reflect the current code conditions.

What needs to be discussed:

• Should the minimal editorconfig be extended to mirror the php-cs-fixer rules when formatting by the IDE? Because currently the align is ignored in the assignments in the editorconfig. I know that PHP-Storm supports this, but I'm not sure about other IDEs.
• Which rules should be enforced with PHP-CS-Fixer?

@aleho fyi

[ovx]

Thanks for the PR, I had to rebase it so merged from another branch.