Make use of ignorePatterns

```
  // ESLint will try to parse these files using TypeScript,
  // but since they're not part of the project, it will complain
  // about them not being listed in tsconfig.json:
  ignorePatterns: [".eslintrc.js", "playwright.config.ts"],
```

Might be worth doing this in core, instead of via `.eslintignore` which is where it's done in other repos.