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))
• 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 ✅

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

Additional details and impacted files

@@           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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