fix (CDVWebViewEngine): JavaScript to Native Call Not Working

[erisu]

Motivation and Context

Fix error when calling WebViewEngine JavaScript Code.

Description

When calling the WebViewEngine JavaScript

window.WkWebView.allowsBackForwardNavigationGestures(true);

The code does not process and an error displays in the Native logs,

2020-02-13 18:51:12.416046+0900 MyAppName[48951:8545412] ERROR: Plugin 'CDVWebViewEngine' not found, or is not a CDVPlugin. Check your plugin mapping in config.xml.
2020-02-13 18:51:12.416177+0900 MyAppName[48951:8545412] FAILED pluginJSON = ["INVALID","CDVWebViewEngine","allowsBackForwardNavigationGestures",[true]]

This PR adds the missing declaration which makes the JavaScript work again.

Testing

• npm t
• cordova platform add
• cordova build ios
• Run Simulator
• Test JavaScript method and verify execution

Checklist

• I've run the tests to see all new and existing tests pass

[jcesarmobile]

Looks good to me