Initial Sponge layer support with what Sponge has added so far to the API.

Some key things are missing:
- Events (naturally)
- Command handling is not implemented. Sponge has command dispatcher classes already, I just don't see anything in Game yet.
- Items don't have enchantment info
- LWC backend doesn't support item names (only indirectly) just yet, so Sponge's ItemType#getId() (String) is not natively supported by LWC yet
Deprecate Engine.startup() and Engine.shutdown()