Expose ItemList's auto height value with get_auto_height_value

[Spartan322]

Relies on #63634

Addresses godotengine/godot-proposals#254 with a more explicit call alongside #63634, as custom_minimum_size may hide the auto height value. This makes reliance on the auto height value more explicit for calculations, however since nothing yet forces the auto_height_value to update regardless of auto_height, technically #63634 (comment) still presents a bit of an issue, as the return value here will just be from the last time auto_height was true or otherwise it'll return 0, perhaps it would be justified to implement:

func update_auto_height(force : bool = false)

With such, an update_auto_height call could force the auto_height_value to update even if auto_height is false as it currently does not allow the user to do so. As it stands the usefulness of this PR is quite limited and only really good for being explicit and avoiding custom_minimum_size.

Exposed Additions

ItemList:

func get_auto_height_value() -> float