fix: undefined fs in iOS

[tralves]

PR Checklist

• [X] The PR title follows our guidelines: https://github.com/Notalib/nativescript-webview-ext/blob/master/CONTRIBUTING.md#commit-messages.
• [X] There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• [X] All existing tests are passing.
  • [X] On Android
  • [X] On iOS 12+
• [X] Tests for the changes are included

What is the current behavior?

Doesn't work on iOS. See #104.

What is the new behavior?

It works on iOS.

Fixes/Implements/Closes #[Issue Number]. Fixes #104.

[tralves]

Replacing with another PR that fixes another bug too.

[tralves]

Replacing with another PR that fixes another bug too.