Improve this repository, add tests

[matks]

This tool is doing amazing things but it can be improved. For example code complexity of class https://github.com/PrestaShop/prestashop-shop-creator/blob/master/src/Generator/EntityGenerator.php is high.

First thing needed to enable refactoring of this repository would be to add automated tests, and run them in a CI using GitHub Actions.

When this is done we will be able to use CI to detect whether our refactoring is breaking an existing feature / behavior or not.

I suppose that we can use this opportunity to add the standard CI helpers: phpstan, php-cs-fixer... see for example https://github.com/PrestaShop/ps_linklist/blob/dev/.github/workflows/php.yml

[mflasquin]

Maybe we can add some steps on this issue with several PR associated :

• Add PHP Linter
• Add php-cs-fixer (with fixes)
• Add phpstan (with fixes)
• Add unit tests