Depends on #3

Replace the LAN ENetMultiplayerPeer with SteamMultiplayerPeer from the GodotSteam. This allows users to connect via Steam Friends/Lobbies without port forwarding

Acceptance Criteria:

•  Game connects to Steam AppID 480 on launch
•  Users can Host a Steam Lobby
•  Users can Join a Steam Lobby (via UI or Steam Friends list)
•  Movement and spawning work exactly as they did in LAN mode