create & export toJSON replacement

[spaghetti-squash]

From discord:

> js JSON.stringify({ x: Item.get(1) }, (_, value) => value instanceof Item ? "fart" : value)

Returned: {"x":{"id":1,"name":"seal-clubbing club","plural":"seal-clubbing clubs","descid":"868780591","image":"club.gif","smallimage":"club.gif","levelreq":0,"quality":"","adventures":"","muscle":"","mysticality":"","moxie":"","fullness":0,"inebriety":0,"spleen":0,"minhp":0,"maxhp":0,"minmp":0,"maxmp":0,"dailyusesleft":2147483647,"notes":"","quest":false,"gift":false,"tradeable":true,"discardable":true,"combat":false,"combatReusable":false,"usable":false,"reusable":false,"multi":false,"fancy":false,"pasteable":true,"smithable":true,"cookable":false,"mixable":false,"candy":false,"candyType":"none","chocolate":false,"potion":false,"seller":"none","buyer":"none","nameLength":18,"noobSkill":"none","tcrsName":"seal-clubbing club","skill":"none","recipe":"none"}}

> js Item.get(1) instanceof Item

Returned: true

it seems like mafia constants get converted to POJOs before hitting the replacer function