Add Illusion to armor traits

[ShelleyKing]

Since Illusion remains a valid trait in the remaster, I think it should be allowed as an armor trait.

(Relatedly, I think the level 13 specific magic armor Harmonic Hauberk should still have this trait using remaster rules, instead of the trait being removed as seems to have happened.)

It looks like Illusion got removed from the list of armor traits in src/scripts/config/traits.ts by commit #11026, because the line ...magicSchools, was removed from const armorTraits.

But the Illusion trait really should have been individually added back in.