✨ duplicate entity instance functionality

[bitbrain]

Description

Sometimes we want to duplicate entities, e.g. when having self-multiplying enemies in the world that already have a custom state. This can be achieved as follows via this PR:

var entity = Pandora.get_entity("123")
var entity_instance = entity.instantiate()
var duplicate_instance = entity_instance.duplicate_instance()

Additionally, you can also call duplicate_instance() on non-instances to create new instances (acts as instantiate()).

Note: the new method is called duplicate_instance() and not duplicate() as that method is already reserved for traditional Godot resources. It has some different meaning/docs so I did not want to mix up these concepts!