Add support for editor-only visibility

[AndrewPrifer]

I have 2 ideas:

1. extend the visibility prop of objects to also accept an "editor-only" string. This will ultimately set the object's visibility to false, unless we are in the editor
2. a separate editorOnly: boolean prop This has come up enough times in my own projects too to warrant a bespoke solution The first one is simpler/makes more sense to me, but might be less intuitive/discoverable because it modifies an existing API