Change "selected" property to "selectedNodeOrEdge"

[ajbogh]

Is your feature request related to a problem? Please describe. The "selected" property can cause some confusion because it doesn't inform the developer if it's a node or edge, or both. This property should change to "selectedNodeOrEdge" to be more clear.

Describe the solution you'd like We want to inform the developers about the expected types through variable or property names as well as type information. In this case "selectedNodeOrEdge" is more clear to the developer than "selected".

The "selected" property can be deprecated in the code and then later removed in the following major release. Deprecation warnings should be included for developers still using "selected".

Additional context https://github.com/uber/react-digraph/issues/240#issuecomment-670979525

[mosofsky]

Renaming selected to selectedNodeOrEdge will help developers learn react-digraph faster and make fewer mistakes like the ones I blundered into in https://github.com/uber/react-digraph/issues/240#issuecomment-670979525.