Support `body` equipment slot (#2736)
* EntityBodyArmor property, `body` argument in `equip` command
* 1.20+ clarified
* slight condition change
* slight correction
* slight fix
* moved body_armor exclusively under `equip` command and as part of `EntityTag.equipment` and `EntityTag.equipment_map`
* added `body_armor` tag to go along with previous `horse_armor`
* deprecated EntityTag.equipment, other minor changes
* removed EntityTag.body_armor
* deprecation file
* minor changes
* deprecation message fix
* -_-
* slight clarification
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commit: 4e6c6f7)
