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/client/UsernameInput.ts

@@ -15,7 +15,7 @@ export class UsernameInput extends LitElement {
   @state() private username = "";
   @property({ type: String }) validationError = "";
   private _isValid = true;
-  private userSettings: UserSettings = new UserSettings();
+  private readonly userSettings: UserSettings = new UserSettings();
 
   // Remove static styles since we're using Tailwind