feature: inventory tracker

[rsek]

Could probably live in a select menu. related: #1

• how do items get added? click on an empty menu slot?
  • modal can prompt for label, description... and maybe emoji!?
• how do items get retrieved (e.g. with a roll)?
• how do items that take up multiple spots get displayed? single entry?
• how do items get rearranged?
• how are stacks of multiple items handled? what about a way to increment/decrement them?
• how are held items handled?