Backport LivingEntity#canUseEquipmentSlot to 1.20.6

[tal5]

Is your feature request related to a problem?

1.20.6 already throws an exception when using BODY for entities that don't have it (672c077), but LivingEntity#canUseEquipmentSlot was only added in 1.21 (which is still experimental).

Describe the solution you'd like.

Would be nice if LivingEntity#canUseEquipmentSlot could be backported to 1.20.6, seeing as it throws for invalid slots but doesn't really have a way to check.

Describe alternatives you've considered.

Could try/catch, but that isn't as nice as a proper method & you can't really know where the IllegalArgumentException is from without checking the message or something.

Other

I know features aren't usually backported like this so I understand if you prefer not to, but imo this specific case makes sense seeing as it's still supported & already has checks to throw for invalid slots and all.

[tal5]

Resolved by #11013