Added loot drops!

- Base chance of receiving a drop from any enemy is 2.5%, which increases as your luck stat increases.

- Items drop in the form of a chest, and when the player collides with the chest, it opens and the item is shown visually, and automatically added to the player's inventory. Keep in mind if you already have a better version of the item, then nothing really happens.

- These drops can occur all the way up to stage 100. 

- When an item drops, its rarity is rolled, although currently rarity doesn't do anything, nor is there any indication for the player to know what rarity their items are. This will be addressed in a future update.

Misc:

- Fixed a bug with bosses not being attackable.