Add "find by selector" support

[srchase]

The IDE will have a menu option to “find by selector”, which will improve the experience for writing selectors for traits or custom validators. When a selector is entered into the dialog, all matching shapes will be highlighted in the model. The same highlighting will be activated when a selector in the model is right-clicked and the “Find by selector” option is clicked on the context menu.

See smithy-lang/smithy-language-server#15 for implementation work on LSP to support this capability.