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

tests/util/Setup.ts

@@ -61,7 +61,7 @@ export async function setup(
     gameMode: GameMode.FFA,
     gameType: GameType.Singleplayer,
     difficulty: Difficulty.Medium,
-    disableNPCs: false,
+    disableNations: false,
     donateGold: false,
     donateTroops: false,
     bots: 0,