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/BotSpawner.ts

@@ -7,11 +7,11 @@ import { TileRef } from "../game/GameMap";
 import { simpleHash } from "../Util";
 
 export class BotSpawner {
-  private random: PseudoRandom;
-  private bots: SpawnExecution[] = [];
+  private readonly random: PseudoRandom;
+  private readonly bots: SpawnExecution[] = [];
 
   constructor(
-    private gs: Game,
+    private readonly gs: Game,
     gameID: GameID,
   ) {
     this.random = new PseudoRandom(simpleHash(gameID));