feat: add item rarity to loot notifications for monsters

[iProdigy]

Partially addresses #139; supersedes #141

Resolves:

• Rare drop table BossHuso/discord-rare-drop-notificater#29
• Nex uniques not showing  BossHuso/discord-rare-drop-notificater#58
• Rarity of loot on multiple tables is not calculated correctly BossHuso/discord-rare-drop-notificater#82
• Rare Drop Notifications at New Bosses BossHuso/discord-rare-drop-notificater#85

Not resolved:

• Grotesque Guardians drops not showing up BossHuso/discord-rare-drop-notificater#26

[pajlada]

Would be nice to include the npc_drops.json source, both as a way for us to update it going forward & as a way to attribute the data

[iProdigy]

Clarified data source in fb477f0

In particular, we feed their parsed data to our script to generate npc_drops.json - will eventually clean this up so it can be ran as an action to automatically update our json file