`EntityTag.is_sneaking` Paper-only property (#2508)
* Add `EntityTag.sneaking` property
* Meta improvements
* Deprecate `PlayerTag.is_sneaking`
* `ItemTag.trim` minor meta fix
* `PlayerTag.is_sneaking` deprecation fix
* Cleanup `MaterialAttachmentFace`
* Revert `PlayerTag.is_sneaking` deprecation
* Register existing tags when the prop isn't present
* Rename to `is_sneaking`
* Update another place
* Bump Paper API, add Citizens dependency
* Properly make NPCs sneak
* Remove trait when standing up
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commit: 19433ee)
