Static door support for mods is lacking

[ajrb]

Describe the issue

Static door (doors that transition between exteriors and interiors) support currently only works for model overrides of vanilla building models with DF door data. Each custom door references a static door on the original model for transitioning inside or back out again. The original doors can be suppressed from being active but the data must exist for a custom door to work. This use of existing static door data also restricts where the custom doors are located to one of the original doors for purposes of finding the closest start marker for positioning player inside.

This means there is currently no way to add doors to custom models, even if they're based on original models - if a custom model id is used, there is no door data available so it doesn't work.

Intention is to improve CustomDoor component.

Improvement todo list

1. Allow custom building models to have transition doors defined that work.
2. Allow custom doors for custom dungeon entrance models.
3. Allow custom interior exit doors for custom interior models.