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
