Goddess bills implementation (custom NPC)

[kenedos]

What this PR Does:

• Adds a new NPC that allows players to exchange their silver for "bill" items.
• Bills can be also converted back to silver in the same NPC.
• With this, players are able to indirectly trade silver with eachother (by exchanging bills, as silver is not normally tradeable).
• In addition to this, this PR includes IES Mods to set the bill item descriptions (optional in items.conf file).
• NPC and conf file option are disabled by default.

What this PR does NOT implement:

• Trading between players