Disable singles without public URIs

[MangoMarcus]

You can currently disable certain entry sources in order to prevent entries without public URLs from being selected, however there's no way to do this for singles as they're all grouped together under "Singles".

This has been an uncommon use case as normally you'd make these global sets instead of singles, however Craft are phasing out global sets as part of their entrification plan. Craft 4.4 includes utilities to migrate global sets to singles, so this use case will be more common going forwards