Option to disable alliances + 2 new modifiers for variety 😄 (#3392)

## Description:

Rex had this idea: "It would be funny to have an option in private
lobbies to disable alliances."
I added it as an option.
Now people can choose to live in constant fear of their neighbors 😆 

Also added two new public game modifiers for variety (only for the
special rotation):
- Alliances disabled (low probability)
- x2 gold multiplier (low probability)

Would be nice to squeeze this into v30, last minute?

## 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/SinglePlayerModal.ts

@@ -56,6 +56,7 @@ const DEFAULT_OPTIONS = {
   startingGold: false,
   startingGoldValue: undefined as number | undefined,
   disabledUnits: [] as UnitType[],
+  disableAlliances: false,
 } as const;
 
 @customElement("single-player-modal")
@@ -90,6 +91,7 @@ export class SinglePlayerModal extends BaseModal {
   @state() private disabledUnits: UnitType[] = [
     ...DEFAULT_OPTIONS.disabledUnits,
   ];
+  @state() private disableAlliances: boolean = DEFAULT_OPTIONS.disableAlliances;
 
   private mapLoader = terrainMapFileLoader;
 
@@ -313,6 +315,10 @@ export class SinglePlayerModal extends BaseModal {
                     labelKey: "single_modal.compact_map",
                     checked: this.compactMap,
                   },
+                  {
+                    labelKey: "single_modal.disable_alliances",
+                    checked: this.disableAlliances,
+                  },
                 ],
                 inputCards,
               },
@@ -383,6 +389,7 @@ export class SinglePlayerModal extends BaseModal {
       this.gameMode !== DEFAULT_OPTIONS.gameMode ||
       this.goldMultiplier !== DEFAULT_OPTIONS.goldMultiplier ||
       this.startingGold !== DEFAULT_OPTIONS.startingGold ||
+      this.disableAlliances !== DEFAULT_OPTIONS.disableAlliances ||
       this.disabledUnits.length > 0
     );
   }
@@ -409,6 +416,7 @@ export class SinglePlayerModal extends BaseModal {
     this.goldMultiplierValue = DEFAULT_OPTIONS.goldMultiplierValue;
     this.startingGold = DEFAULT_OPTIONS.startingGold;
     this.startingGoldValue = DEFAULT_OPTIONS.startingGoldValue;
+    this.disableAlliances = DEFAULT_OPTIONS.disableAlliances;
   }
 
   protected onOpen(): void {
@@ -488,6 +496,9 @@ export class SinglePlayerModal extends BaseModal {
       case "single_modal.compact_map":
         this.handleCompactMapChange(checked);
         break;
+      case "single_modal.disable_alliances":
+        this.disableAlliances = checked;
+        break;
       default:
         break;
     }
@@ -696,6 +707,7 @@ export class SinglePlayerModal extends BaseModal {
                     ),
                   }
                 : {}),
+              ...(this.disableAlliances ? { disableAlliances: true } : {}),
             },
             lobbyCreatedAt: Date.now(), // ms; server should be authoritative in MP
           },