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/client/HostLobbyModal.ts

@@ -38,7 +38,7 @@ export class HostLobbyModal extends LitElement {
   };
   @state() private selectedMap: GameMapType = GameMapType.World;
   @state() private selectedDifficulty: Difficulty = Difficulty.Medium;
-  @state() private disableNPCs = false;
+  @state() private disableNations = false;
   @state() private gameMode: GameMode = GameMode.FFA;
   @state() private teamCount: TeamCountConfig = 2;
   @state() private bots: number = 400;
@@ -358,17 +358,17 @@ export class HostLobbyModal extends LitElement {
                   )
                     ? html`
                         <label
-                          for="disable-npcs"
-                          class="option-card ${this.disableNPCs
+                          for="disable-nations"
+                          class="option-card ${this.disableNations
                             ? "selected"
                             : ""}"
                         >
                           <div class="checkbox-icon"></div>
                           <input
                             type="checkbox"
-                            id="disable-npcs"
-                            @change=${this.handleDisableNPCsChange}
-                            .checked=${this.disableNPCs}
+                            id="disable-nations"
+                            @change=${this.handleDisableNationsChange}
+                            .checked=${this.disableNations}
                           />
                           <div class="option-card-title">
                             ${translateText("host_modal.disable_nations")}
@@ -556,7 +556,7 @@ export class HostLobbyModal extends LitElement {
                 : translateText("host_modal.players")
             }
             <span style="margin: 0 8px;">•</span>
-            ${this.disableNPCs ? 0 : this.nationCount}
+            ${this.disableNations ? 0 : this.nationCount}
             ${
               this.nationCount === 1
                 ? translateText("host_modal.nation_player")
@@ -569,7 +569,7 @@ export class HostLobbyModal extends LitElement {
             .clients=${this.clients}
             .lobbyCreatorClientID=${this.lobbyCreatorClientID}
             .teamCount=${this.teamCount}
-            .nationCount=${this.disableNPCs ? 0 : this.nationCount}
+            .nationCount=${this.disableNations ? 0 : this.nationCount}
             .onKickPlayer=${(clientID: string) => this.kickPlayer(clientID)}
           ></lobby-team-view>
         </div>
@@ -735,9 +735,9 @@ export class HostLobbyModal extends LitElement {
     this.putGameConfig();
   }
 
-  private async handleDisableNPCsChange(e: Event) {
-    this.disableNPCs = Boolean((e.target as HTMLInputElement).checked);
-    console.log(`updating disable npcs to ${this.disableNPCs}`);
+  private async handleDisableNationsChange(e: Event) {
+    this.disableNations = Boolean((e.target as HTMLInputElement).checked);
+    console.log(`updating disable nations to ${this.disableNations}`);
     this.putGameConfig();
   }
 
@@ -779,10 +779,10 @@ export class HostLobbyModal extends LitElement {
           ...(this.gameMode === GameMode.Team &&
           this.teamCount === HumansVsNations
             ? {
-                disableNPCs: false,
+                disableNations: false,
               }
             : {
-                disableNPCs: this.disableNPCs,
+                disableNations: this.disableNations,
               }),
           maxTimerValue:
             this.maxTimer === true ? this.maxTimerValue : undefined,