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

tests/Censor.test.ts

@@ -2,7 +2,7 @@
 jest.mock("obscenity", () => {
   return {
     RegExpMatcher: class {
-      private dummy: string[] = ["foo", "bar", "leet", "code"];
+      private readonly dummy: string[] = ["foo", "bar", "leet", "code"];
       constructor(_opts: any) {}
       hasMatch(input: string): boolean {
         const lower = input.toLowerCase();