BattleArena

Add a BattleArenaApi class

This is intended to be a stable entrypoint into BattleArena. While most of the plugin is designed to be safe regardless of whether it is "API" or not, this is a safer option for third parties in the event major changes need to occur internally.

This also allows a slight bit more flexibility with what exists in the main BattleArena class, as its functionality should be depended on less by implementing plugins. The former BattleArena.getInstance() entrypoint is not deprecated and offers a handful more methods, but should only be used in events where that is needed.