PHP 8.1 - 8.3 + Docker + Github Actions + Refactoring of the code

[bytes-commerce]

This PR will complete deprecate PHP7 usage as it was possible years before, as PHP7 is already a long time end of life.

Previous branches can be used if PHP7 is planned to be kept alive.

This PR is also not backwards compatible. I've changed quite some semantics to remove most of the smelly code (i.e. transmit arbitrary strings to functions that require a specific format in order to work as expected).

This PR introduces more things:

• Docker Compose env
• Github Actions for PHP 8.1, 8.2 and 8.3 (!)
• Phpstan
• ECS (PHPCS on Steroids)

Coverage is around ~75%.

Tests are running fine. Scrutinizer is failing to build, but GitHub Actions are in good conditions, see my own PR in my fork: https://github.com/nopenopenope/vcard/actions/runs/7450641541/job/20270001192