Add Demon Aura

[cpaca]

Tested:

• Getting the book from the recipe
• Getting the book from /holoitems acquire
• Using either book to apply ench on armor
• Having more armor pieces means getting more health.

Not Tested:

• WorldGuard Integration

From what I can tell, it's always half a heart from a 1-xp orb (regardless of armor pieces), but larger orbs and more armor pieces gives more hp.
With a 100-xp orb and starting at 0 xp (and health boost 100):
One armor piece gives 7.5 hearts and 6.75-ish levels.
Four armor pieces gives 24(ish) hearts and 4.75-ish levels

Questions (these are in the code, in the TODOs, but I'll paste them here):

• Do we want DAura on elytras? Both old and current implementations don't allow it.
• What should the cost multiplier be? (For anvils)? I used 12, but that's copying from magnet
• Should we assume 20 max hp or use Attribute.GENERIC_MAX_HP? The first one is what OldHoloItems uses, but the second one is what I used.

[dlee13]

Blocked on Paper API 1.21
https://jd.papermc.io/paper/1.21/org/bukkit/entity/Damageable.html#heal(double)