server:Add game event handler for JOIN_GAME event

This commit creates the event handler for JOIN_GAME event to let new
players join the server before the game is started by the host.

Fixes:shivansh-bhatnagar18#93

backend/src/uno-game-engine/events/joinGame.ts

@@ -0,0 +1,12 @@
+import { assert } from 'console';
+import { GameEngine } from '../engine';
+import { registerEventHandler } from '../gameEvents';
+
+export function joinGame(game: GameEngine, event: GameEvent): EventResult {
+    assert(event.type === 'JOIN_GAME', 'Invalid event type');
+    const player: Player = { id: event.playerId, cards: [] };
+    game.addPlayer(player);
+    return { type: 'SUCCESS', message: 'player joined successfully' };
+}
+
+registerEventHandler('JOIN_GAME', joinGame);