Switching to definitely assigned parameters

This allows us to go in "full strict mode" (yeah!)
See https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-7.html#strict-class-initialization

front/src/Phaser/Game/GameManager.ts

@@ -13,8 +13,8 @@ export interface HasMovedEvent {
 }
 
 export class GameManager {
-    private playerName: string;
-    private characterLayers: string[];
+    private playerName!: string;
+    private characterLayers!: string[];
 
     public setPlayerName(name: string): void {
         this.playerName = name;