Feature: Implemented Selection#is() and DocumentSelection#is() methods.

[oleq]

Suggested merge commit message (convention)

Feature: Implemented Selection#is() and DocumentSelection#is() methods in both the model and the view. Closes ckeditor/ckeditor5#4477.

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A piece of https://github.com/ckeditor/ckeditor5-engine/issues/1667. To get rid of instanceof checks we must implement is() in other classes too.

[pjasiun]

I think you should add the same methods to engine#view.selection and engine#view.documentselection

[coveralls]

Coverage remained the same at 100.0% when pulling b81e921 on t/1663 into c9932b8 on master.

[pjasiun]

I think that to close #1663 this ticket should also replace usage of instanceof with new is method.

Nvmd. Lets close #1667 first to be safer.