Define nether portal destinations in custom dimension types

Currently, nether portals are not allowed within custom dimensions.

Instead, dimension types (data/<namespace>/dimension_type/<name>.json) should be able to define the destination dimension which nether portals would lead to. This would be an additional property "nether_portal_destination", with a value defining the namespaced destination dimension name (eg: "minecraft:overworld")

If a destination is not specified, nether portals would be disabled (as they are now for all custom dimensions).

There are several other similar suggestions which indicate a desire for creating entirely custom portals. This suggestion is a simpler option to give us support to use the portals we already have. For example, it would enable us to have "nether" portals within the nether lead to a third dimension, with that third dimension leading back to the overworld. Or multiple custom dimensions with their own psuedo-nether/pseudo-overworld pairs, etc.