Add support for slotted attributes

[Pablete1234]

Fixes #1446

Allows attributes to specify a slot that the item has to be in to work:

<attribute amount="3" slot="mainhand">generic.attackDamage</attribute>

Note: This will only work when running in modern paper, legacy (1.8) minecraft didn't have support for this.

Slots can be specified using EquipmentSlotGroup which is the modern way, allowing for things like armor (for any armor piece) or hand (for either hand), but it also, for backwards-compat reasons, supports slots too like slot.armor.head.

Modern values (recommended)

any -> any equipment slot, the default if you don't set one
mainhand
offhand
hand  -> either main or off-hand
feet
legs
chest
head
armor -> any armor piece
body -> Used for entities with no feet/legs/chest/head distinction (eg: horses when they wear armor)

Legacy (deprecated) slots

They only work for backwards-compat reasons, and realistically do not need to be documented, focus on documenting the above ones instead.

All of these may have the slot. prefix, or not, like weapon.mainhand or slot.weapon.mainhand:

weapon.mainhand
weapon.offhand
armor.feet
armor.legs
armor.chest
armor.head

[cswhite2000]

Interesting use of composition