Removes some finished TODOs
Registers all listeners
Registers all commands
Adds more descriptive TODO comments to each command
Removes unintended double storage of dropper arena sessions in DropperArenaHandler
Adds commands and permissions to plugin.yml
Uses the Spigot API instead of the Paper API
Reduces the Java version to 16, just in case.
Adds a lot of empty classes to show the intended plugin structure.
