[FEATURE] Sync existing projects

[eliashaeussler]

Is your feature request related to a problem?

The current implementation has only one feature, that is, creating new projects from scratch. However, it might also become handy to sync existing projects with the latest version of the project template that was previously used to create the project.

Describe the solution you'd like

We should provide a command to sync existing projects. This requires to decouple the project creation logic from the actual single use case with composer create-project. In addition, a build artifact (as requested in #12) is required in order to provide the initial project state to create a meaningful diff.

Describe alternatives you've considered

No response

Additional context

Learn from the best – we should have a look at how Symfony Flex does it similarly with updating recipes. A good starting point might be https://github.com/symfony/flex/blob/1.x/src/Update/RecipePatcher.php.

It's probably also worth to have a look at https://github.com/spatie/package-sync.