Cleanup nations (Part 1) 🧹 (#2637)

## Description:

1. Using the wording `"Nation"`, `"FakeHuman"` and `"NPC"` at the same
time is confusing.
So I renamed every mention of `"FakeHuman"` and `"NPC"` in the entire
project to `"Nation"`. Just like they are called ingame.

2. `BotBehavior.ts` was originally intended for sharing the logic
between nations and bots.
But at the moment, the logic there isn't really shared and it's
basically just about attacking.
So I renamed `BotBehavior.ts` to `AiAttackBehavior.ts`. I use "Ai" to
indicate that this file is used by bots AND nations.

3. Moved `execuction/utils/AllianceBehavior.ts` to
`execuction/nation/NationAllianceBehavior.ts` to make sure everybody
understands that this file is not about alliances in general. It's just
about nations and how they handle alliances.

4. Removed `difficultyModifier` from `DefaultConfig`. It's unused and I
think we usually want to finetune the difficulty instead of using that
method.

5. Added `assertNever` in all `switch (difficulty)` default cases.

## 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/core/game/TeamAssignment.ts

@@ -59,7 +59,7 @@ export function assignTeams(
 
   // Then, assign non-clan players to balance teams
   let nationPlayers = noClanPlayers.filter(
-    (player) => player.playerType === PlayerType.FakeHuman,
+    (player) => player.playerType === PlayerType.Nation,
   );
   if (nationPlayers.length > 0) {
     // Shuffle only nations to randomize their team assignment
@@ -67,7 +67,7 @@ export function assignTeams(
     nationPlayers = random.shuffleArray(nationPlayers);
   }
   const otherPlayers = noClanPlayers.filter(
-    (player) => player.playerType !== PlayerType.FakeHuman,
+    (player) => player.playerType !== PlayerType.Nation,
   );
 
   for (const player of otherPlayers.concat(nationPlayers)) {