Added Britannia map with 23 NPC as well as their flags (#186)

Let me know if you have any questions or concerns. This is my first time
near coding, so it would not hurt to double check my work. Thanks!

src/core/game/Game.ts

@@ -49,6 +49,7 @@ export enum GameMapType {
   Africa = "Africa",
   Asia = "Asia",
   Mars = "Mars",
+  Britannia = "Britannia",
 }
 
 export enum GameType {

src/core/game/TerrainMapFileLoader.ts

@@ -34,6 +34,7 @@ const MAP_FILE_NAMES: Record<GameMapType, string> = {
   [GameMapType.Asia]: "Asia",
   [GameMapType.Mars]: "Mars",
   [GameMapType.SouthAmerica]: "SouthAmerica",
+  [GameMapType.Britannia]: "Britannia",
 };
 
 class GameMapLoader {