Add Halkidiki Map (#799)

## Description:
This PR adds the Halkidiki map. It's a region in Greece with rich
history that is very popular also with tourism.

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

## Please put your Discord username so you can be contacted if a bug or
regression is found:

<DISCORD USERNAME>
Nikola123

src/core/game/TerrainMapFileLoader.ts

@@ -46,6 +46,7 @@ const MAP_FILE_NAMES: Record<GameMapType, string> = {
   [GameMapType.EuropeClassic]: "EuropeClassic",
   [GameMapType.FalklandIslands]: "FalklandIslands",
   [GameMapType.Baikal]: "Baikal",
+  [GameMapType.Halkidiki]: "Halkidiki",
 };
 
 class GameMapLoader {