Allow instantiating scenes without any scripts

[ryevdokimov]

Closes: godotengine/godot-proposals#12647

This would be useful when need a scene for purely visual elements such as generating thumbnails or creating decorative versions.

We already have the ability the include flags to remove scripts when duplicating, so this just removes the extra step in a performant way.

[RedMser]

The functionality seems useful, but I'm unsure about putting it into GenEditState. It's currently an editor-only enum, and you're adding an unrelated entry to it that also has a runtime effect.
Furthermore, it feels like it should be a bitflag like is the case with duplicate, but this is just a regular enum so you couldn't combine this value with others.

I think this needs a proper design proposal first. https://github.com/godotengine/godot-proposals
Simplest solution could be a new flags parameter.

[ryevdokimov]

Agreed, I've updated the PR to separate it out and will make a dedicated proposal.

[RedMser]

Docs should likely mention whether this affects child scenes as well, or only nodes from its own scene. (Same goes for SceneState docs).

Addressed

[RedMser]

As far as I can tell, this is not needed:

// Note: In most situations, compatibility methods should be registered via ClassDB::bind_compatibility_method().
//       This class is only meant to be used in exceptional circumstances, for example, when Godot's hashing
//       algorithm changes and registering compatibility methods for all affect methods would be onerous.