Allow users to hide the map they're in

[SnipUndercover]

This feature will allow people to hide the map they're in. Useful for mapping in secret without having to worry about leaking map names (or for people who hate socializing).

[SnipUndercover]

Force pushed to remove an accidental CelesteNet.props commit to let me compile on core Everest :gladbw: Will re-add the commits soon.

[RedFlames]

Looking at PRs, was wondering why this was still "draft" and has been collecting dust for quite a while... (well, moreso than everything else) and why was it me that set it to draft?...

Turns out we spoke about it briefly on Discord that day, and the issue here is that the server uses SID comparisons to determine when players are in the same level. And so if a player client is hiding that information, it looks to the server as if they're on main menu, and can't get/send Ghost updates etc.

The relevant bits are CelesteNetPlayerSession.IsSameArea which is only used by CelesteNetPlayerSession.Handle(CelesteNetConnection con, DataType data) to determine which players to broadcast MetaPlayerUpdate data to.
And the Client has some SID comparisons too for ghost update related things.

If we want to fix this issue, the clients with hidden location / SID would still need to send it to the server, so that the server can do the "same area" checks normally, and then mask out the info in DataPlayerState but only when sending it to players who are not in the same area? 🤔