Not having prefer-const on the JS side makes JS -> TS refactors really
unreadable. This commit just auto-fixes it so we're using
`prefer-const` everywhere.
By adding a `getGame` function of type:
```
// null is returned if the gameids don't match
// or the game doesn't exist
getGame<T extends RoomGame>(constructor: new (...args: any[]) => T) => T | null
```
(Credits @urkerab and @whalemer for the function signature.)
It allows refactoring previous code of:
```
if (room.game && room.game.gameid !== 'hangman') return;
const game = room.game as Hangman;
```
to:
```
const game = room.getGame(Hangman);
if (!game) return;
```
This has a couple of advantages:
- TypeScript will throw an error if the if condition is not present.
- In the new code, the template must extends `RoomGame` and be assignable to the same ID, so it's 100% typesafe
- The big change here is that player.userid can now be empty. You can
now fit state into RoomGamePlayer subclasses even when there are no
users associated with them.
- `game.players` has been introduced as the new canonical list of
players, including userless players. The old `game.players` has been
renamed `game.playerTable`, for clarity.
- `game.addPlayer` now returns the added player
All existing RoomGames have been updated for the new API, and
RoomBattle is now officially a RoomGame subclass.
Tournaments was also massively refactored to be properly updated for
the old API, since that never happened, and should now be a lot more
readable.
