Some little HumansVsNations improvements before public games launch 🤖 (#2825)

## Description:

- Added `generateUniqueNationName()` to `NationCreation` because I saw a
duplicate name while spawning 300 nations on Pangaea 😄
- Switched HumansVsNations public game difficulty from hard to
impossible because I realized how crazy strong troop donations between
humans are (in an enzo HVN stream).

Maybe I have to make nations donate troops to each other, we will see...
Playtests won't tell the truth because the players attending these are
probably better than the usual OF player. I will try to check the HVN
winrate via API after launch

## 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
- [X] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

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

FloPinguin

src/server/MapPlaylist.ts

@@ -121,7 +121,9 @@ export class MapPlaylist {
       gameMapSize: isCompact ? GameMapSize.Compact : GameMapSize.Normal,
       publicGameModifiers: { isCompact, isRandomSpawn },
       difficulty:
-        playerTeams === HumansVsNations ? Difficulty.Hard : Difficulty.Easy,
+        playerTeams === HumansVsNations
+          ? Difficulty.Impossible
+          : Difficulty.Easy,
       infiniteGold: false,
       infiniteTroops: false,
       maxTimerValue: undefined,