Enable the @typescript-eslint/prefer-readonly eslint rule (#1859)

## Description:

Enable the `@typescript-eslint/prefer-readonly` eslint rule.

## 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

src/core/execution/alliance/AllianceRequestExecution.ts

@@ -5,8 +5,8 @@ export class AllianceRequestExecution implements Execution {
   private recipient: Player | null = null;
 
   constructor(
-    private requestor: Player,
-    private recipientID: PlayerID,
+    private readonly requestor: Player,
+    private readonly recipientID: PlayerID,
   ) {}
 
   init(mg: Game, ticks: number): void {