Adds Mississippi River map - vertical pipe map (#4176)

Resolves #4153 

## Description:

Add Mississippi river, inspired by Amazon, but vertical. Pipe-type map
along Amazon and Passage. 11 nations, with 51 additional nations for a
total of 62 for Humans vs Nations gamemode.


https://github.com/user-attachments/assets/6596a7bf-b529-442a-99b1-815493ee0e96


https://github.com/user-attachments/assets/5bb4959b-8ef3-428a-8e3a-94c424fa092b


https://github.com/user-attachments/assets/e4d4622e-ea42-4edf-9d86-d9d00c0fdde4

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory

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

tri.star1011

src/core/game/Game.ts

@@ -183,6 +183,7 @@ export enum GameMapType {
   Onion = "Onion",
   ChoppingBlock = "Chopping Block",
   SoutheastAsia = "SoutheastAsia",
+  MississippiRiver = "Mississippi River",
 }
 
 export type GameMapName = keyof typeof GameMapType;
@@ -255,6 +256,7 @@ export const mapCategories: Record<string, GameMapType[]> = {
     GameMapType.YellowSea,
     GameMapType.Caribbean,
     GameMapType.SoutheastAsia,
+    GameMapType.MississippiRiver,
   ],
   fantasy: [
     GameMapType.Pangaea,

src/server/MapPlaylist.ts

@@ -85,6 +85,7 @@ const FREQUENCY: Partial<Record<GameMapName, number>> = {
   Mena: 6,
   MiddleEast: 8,
   MilkyWay: 8,
+  MississippiRiver: 3,
   Montreal: 6,
   NewYorkCity: 3,
   NileDelta: 4,