restructured Gradle projects

[ErwinOlie]

• moved the autotest-runner project inside /lib/
• introduced single root project with many subprojects
• centralized the settings.gradle.kts and Gradle wrapper files

besides only having to maintain a single Gradle root projects, this opens up the posibility to use version catalogs and fixes IntelliJ syntax highlighting of the .kts files.

[github-actions]

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[ErwinOlie]

@ErikSchierboom ready for review

[ErikSchierboom]

I'll review tomorrow

[ErikSchierboom]

I'm not entirely sure you want to delete the gradle files from the examples. They will be there when the test runner processes a submitted solution. That said, I don't think the test runner uses them, as we're using Maven to run the tests. So it might be fine. Just something to keep in mind.