fix: WKWebView leak in the platform centered approach

[msmtamburro]

Platforms affected

iOS

Motivation and Context

Addresses #1486

Description

Simply clears the _webViewEngine stored property in this class on ‘dispose’ so that it no longer retains a reference to the WKWebView after dispose.

Testing

In the platform centered approach (e.g, adding cordova-ios to an otherwise native app) via a swift package, we can subclass CDVViewController and dispose all plugins when our ViewController is no longer in the view hierarchy.

Checklist

• [ ] I've run the tests to see all new and existing tests pass
• [ ] I added automated test coverage as appropriate for this change
• [ ] Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• [x] If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• [ ] I've updated the documentation if necessary

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 80.86%. Comparing base (2f0e19e) to head (9ceadf4).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1497 +/- ## ======================================= Coverage 80.86% 80.86% ======================================= Files 16 16 Lines 1840 1840 ======================================= Hits 1488 1488 Misses 352 352 ```

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