Enhance LayerList Widget Legend for better UX with Unsupported Layer Types

[KarlLarsen]

Certain layers in the LayerList Widget do not render a legend and instead say 'No Legend' after the user expands the Legend button. Enhance the LayerList Widget to remove or grey out the Legend button when No Legend exists to improve user experience. 

For reference, still as of JS API 4.24, on https://developers.arcgis.com/javascript/latest/api-reference/esri-widgets-Legend.html#hideLayersNot... it says:

Known Limitations

• Currently, the legend widget does not support the following layer types: ElevationLayer, GraphicsLayer, IntegratedMeshLayer, KMLLayer, MapNotesLayer, OpenStreetMapLayer, VectorTileLayer, and WebTileLayer.
• 3D symbols with more than one symbol layer are not supported.
• DictionaryRenderer is not supported.

[KristianEkenes]

This is a good idea. I've logged an enhancement request to implement this behavior. Thanks for the feedback.