Add possibility to select changes for new version

[nerdcel]

Describe the PR

Instead of automatically add (stage) all changes to be in a new created version, the user can now select which changes should be versioned. #4

• Added unit tests for fixed bug/feature
• Added in-code documentation (if needed)
• CI passes (runs automatically when the PR is created or run composer ci locally)
  Running locally requires PHPUnit, PHP-CS-Fixer, Psalm, PHPCPD and PHPMD.

[lukasbestle]

Thank you for your PR and sorry for the delay. There is a lot going on at the moment, so it will unfortunately still take a while until I can review it.

[lukasbestle]

This looks promising and I agree that staging individual files is a useful feature. I'd implement a few things differently and wanted to get v4 support out of the door first.

[lukasbestle]

Now I had the chance to look at this in more detail. Thanks for your work. 💛

Unfortunately I cannot merge this as is, mainly for two conceptual reasons:

• The current setup relies on the prepareVersionCreation step as Git only fully populates the list of changes once they are in the index. E.g. renamed files are only detected as such once both the "added" and "removed" files are in the index. So basing the UI on the state before adding anything to the index won't match the expected UX of common Git clients. They also start with the simple view of changes, but e.g. once you add a directory to the index, the view updates. That is currently not possible without a live-updating list of changes. But I'm not sure if such a live-updating list is worth it complexity-wise.
• The backend needs to validate the list of files to stage. Otherwise one could easily execute different commands via argument injection.

At the moment I don't have the resources to work on the feature further and to be honest I also cannot guarantee that I can review external code for this plugin as I wrote in the contributing guide.