`processing/java/gradle` Rework libraries import

Related https://github.com/processing/processing4/pull/1233

The current Processing Gradle plugin imports Processing libraries successfully, but only if the processing.sketchfolder is defined and Gradle 8 is used.

The plugin will supposedly stop working with Gradle 9 as support for adding dependencies after evaluation has been deprecated. (Mutating a configuration after it has been resolved, consumed as a variant, or used for generating published metadata. This behavior has been deprecated.).
Furthermore, because the dependencies of the project are defined within the .pde files we get a catch 22, either we parse the .pde files on plugin evaluation (which is not where that is supposed to be happening and breaks only running on changes) or we move the dependency declaration out of the .pde files. Neither is ideal, so more research is needed.

As for the sketchbook, it would be ideal if the plugin could find the sketchbook location on its own, but that would require finishing the refactoring work of the settingsFolder path and the sketchbookFolder into app:utils as those variables are currently locked behind running the PDE in its entirety.

Beginne mit der Prüfung des zugehörigen PR 1233 und des Plugins unter processing/java/gradle. Untersuche die Deprecation der Dependency-Evaluierung in Gradle 9, wie .pde-Dateien Dependencies definieren, und das Refactoring von settingsFolder/sketchbookFolder in app:utils. Als erledigt gilt die Aufgabe, wenn Libraries importiert werden können, ohne processing.sketchfolder zu benötigen, und das Plugin mit den unterstützten Gradle-Versionen kompatibel bleibt.