Add top-level @export to separate class_name.

[willnationsdev]

Closes godotengine/godot-proposals#4493.

Depends on #62411. Can be rebased after it is merged.

It also involves a change to the filesystem cache just like #60606.

Modifies @export annotation in GDScript so that it can be used similarly to @tool, but for the purpose of exposing a given class to the editor. Without it, the class will not be visible in the CreateDialog or EditorResourcePicker. This would be a compatibility-breaking change needed for 4.0 IF we do indeed want the default behavior of class_name to not include editor exposure.

Under the hood, I've added a GlobalScriptFlags enum to global script class information stored by the ScriptServer and EditorFileSystem cache. It can have - for now - either GLOBAL_SCRIPT_NONE or GLOBAL_SCRIPT_EXPORT, with the potential for additional flags to be added over time (perhaps GLOBAL_SCRIPT_ABSTRACT or GLOBAL_SCRIPT_STATIC, etc.). Adding the @export class annotation merely changes what value is exposed for the flags value passed to ScriptServer::get_global_class_name(...).

Edit: I noticed that there was a template for creating VisualScriptNodes using GDScript that relies on class_name definitions. Should that functionality also be dependent on the given GDScript having the @export class annotation?

[aaronfranke]

See also my comment here which notes how adding @abstract solves this problem for many of the use cases.

[dalexeev]

I'm not sure if it's a good idea to reuse the @export annotation for this purpose, I would prefer a separate annotation.

Also, this PR implies that by default classes are not displayed in the nodes dialog? Formally, this breaks compatibility. This only affects the editor, though. The alternative is an annotation to hide nodes from the dialog (and by default the current behavior is saved).