Multi world support

[NotAlexNoyle]

• Give each multi world game world its own chat, so players in a game only talk to peers.
• Link all of a multi world game's worlds to a single Discord channel, set up under a new discord.games config section.
• Label each world in Discord so its messages are easy to tell apart, like [HB1 - Lobby] and [HB1 - In Game].
• Send joins, quits, kicks, deaths and advancements to the same channel as the player's world's chat. The main world, the nether and the end use the main chat.
• Announce players in a game's channel when they are teleported in or out, so player count in those messages stays correct.
• Add an API so that a multi world game plugin can style its own chat. Chat-OG still decides who receives which messages, in order to make the API simple for consumers.
• Fixed an unrelated bug where a message was being sent to Discord even when the blocklist had already blocked it in game.

Co-authored by Claude Opus 5