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/PatternDecoder.ts

@@ -1,5 +1,5 @@
 export class PatternDecoder {
-  private bytes: Uint8Array;
+  private readonly bytes: Uint8Array;
 
   readonly height: number;
   readonly width: number;