LSP: Fix singleton registration

[HolonProduction]

Fixes #52162
Fixes #86132

The singleton is now registered on MODULE_INITIALIZATION_LEVEL_EDITOR instead of a late callback.

To make this work I removed an old test case that limited singletons to only types that inherit directly from Object. This should be fine based on my question on RC.

[Repiteo]

Discussed in this rc thread, and I agree that singletons should not be constrained to Object alone (at least, not without it being explicitly documented as such).

[Ivorforce]

Agreed.
I can't think of a reason to restrict to direct Object succession. It might just be leftover code from when it made more sense.

The rest of the changes also look good to me.

[Repiteo]

Thanks!