feat: add support for Java 25 and rework some entity relationships

[theEvilReaper]

Proposed changes

The pull request updates the used version of Java to 25 and reworks the relationship definition in the ItemModel and FontModel. Now they are separated entity and not just element collections

Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

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• I have read the CONTRIBUTING.md
• I have added tests that prove my fix is effective or that my feature works
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