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/ShellExecution.ts

@@ -12,10 +12,10 @@ export class ShellExecution implements Execution {
   private random: PseudoRandom;
 
   constructor(
-    private spawn: TileRef,
-    private _owner: Player,
-    private ownerUnit: Unit,
-    private target: Unit,
+    private readonly spawn: TileRef,
+    private readonly _owner: Player,
+    private readonly ownerUnit: Unit,
+    private readonly target: Unit,
   ) {}
 
   init(mg: Game, ticks: number): void {