feat: add item group key to item group items

[ybw0014]

Description

ItemGroup items can be arbitrary. A Slimefun item, a simple renamed vanilla item, or a complex item can be an ItemGroup item. It would be good if they could have their NamespacedKey in their PDC so other plugins can read this as an ItemGroup item.

Proposed changes

ItemGroup items now have slimefun:item_group in their PDC.

Related Issues (if applicable)

Checklist

• [x] I have fully tested the proposed changes and promise that they will not break everything into chaos.
• [ ] I have also tested the proposed changes in combination with various popular addons and can confirm my changes do not break them.
• [ ] I have made sure that the proposed changes do not break compatibility across the supported Minecraft versions (1.16. - 1.20.).
• [ ] I followed the existing code standards and didn't mess up the formatting.
• [ ] I did my best to add documentation to any public classes or methods I added.
• [ ] I have added Nonnull and Nullable annotations to my methods to indicate their behaviour for null values
• [ ] I added sufficient Unit Tests to cover my code.

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fix/**	✨ Fix
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performance/**	💡 Performance Optimization
compatibility/**	🤝 Compatibility

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[github-actions[bot]]

Slimefun preview build

A Slimefun preview build is available for testing! Commit: 08574465

https://preview-builds.walshy.dev/download/Slimefun/4158/08574465

Note: This is not a supported build and is only here for the purposes of testing. Do not run this on a live server and do not report bugs anywhere but this PR!

[ybw0014]

What's the use case for the PDC here?

For my addon SlimefunTranslation. Slimefun's localization only covers Slimefun's ItemGroups, it is a bit annoying to add localization for addons' ItemGroups. (unless I missed something)