Make export nodes work with custom node types

[nathanfranke]

Export node types: #1048
Export custom resources (similar): #18

Describe the project you are working on

RTS with a lot of export nodes to link properties (e.g. the owner of an entity is a player node).

Describe the problem or limitation you are having in your project

I can't limit the class of an export node to my custom class.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Allowing users to export custom node classes reduces the chance of mistakes when a specific node is expected in a variable. Also helps a ton with auto-completion.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

For example, in my entity class, I can have

@export var player: Player

If this enhancement will not be used often, can it be worked around with a few lines of script?

Will be used often. Can only be worked around with an additional variable.

Is there a reason why this should be core and not an add-on in the asset library?

GDScript is core

[akien-mga]

Implemented by godotengine/godot#67055.