Widget Visibility: Individual custom posts should be listed too, not just the individual regular pages

There currently exists the ability to show a widget on a specific page or on all custom posts of a given type, but you can't show a widget on a specific single custom post. You should be able to.

No file, test, or entry point is named. Start by locating the existing widget visibility logic for specific regular pages and all custom posts of a type, then trace how those cases are represented. Done means a widget can also target one specific custom post without changing the existing page or post-type behavior.

Written by the indexing model from the issue text.