Added isDisplayed() as an alias command for isVisible() in new Element API and added extra test cases

[Ritika8081]

solved #4105
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[Ritika8081]

@garg3133 please have a look Added isDisplayed() as an alias command for isVisible() in new Element API and added extra test cases

[garg3133]

@Ritika8081 Thanks, but the changes need to be done in the main codebase of Nightwatch (present inside lib directory) and not in the example tests provided by Nightwatch.

But we already have a PR #4107 with the correct solution for the issue, so closing this one. Thanks!