public Selector getRootShapeSelector() {
            return rootShapeSelector;
        }
The Selector used to find root shapes, and any shape that is not connected to any of the returned root
shapes is considered unreferenced.

<p>Defaults to "service" if not set.

@return Returns the root selector;
smithy-model/src/main/java/software/amazon/smithy/model/validation/linters/UnreferencedShapeValidator.java