Prevent instantiating classes that aren't exposed

[dsnopek]

Originally from #99534

Currently, if you call ClassDB::can_instantiate("...") for an internal class, it will return true, but most normal ways of instantiating the class in GDScript, GDExtension or C# won't work. However, they still can be created via via ClassDB::instantiate("..."). There's also no indication to GDExtension or scripting from ClassDB that these classes can't be instantiated normally because we don't expose ClassDB::class_is_exposed().

For example:

func _ready() -> void:
	# Prints: true
	print(ClassDB.class_exists("AnimationNodeStartState"))

	# Prints: true
	print(ClassDB.can_instantiate("AnimationNodeStartState"))

	# Prints: <AnimationNodeStartState#12345>
	print(ClassDB.instantiate("AnimationNodeStartState"))

	# ERROR!
	AnimationNodeStartState.new()

Do we even want internal classes to be instantiatable via ClassDB::instantiate("...")?

This PR answers no, and makes it so that internal classes return false for ClassDB::can_instantiate() and can't be created via ClassDB::instantiate("...").

I don't think there's anything that depends on the old behavior, but I'm not 100% sure, so treat this one with caution.

[RandomShaper]

Following you latest comment on the issue, I agree with this approach.

Aside, I'm wondering if we're using two different terms to mean the same across the API and implementation: "internal" and "not exposed". If that's true, we may want to harominze the terminology.