Data pack dimensions can only have their size configured on a single axis

Likely a feature request but an obvious inconsistency nonetheless.

When defining a new dimension via a data pack, only the buildable size of the Y axis can be configured (via min_y and height) - the x and z axes remain completely hardcoded at 30,000,000. This can be undesirable for dimensions intended to only have a limited size, as well as rather limiting in several other aspects.