Create a test to prevent serialization bugs with new settings

[calherries]

This bug #36232 happened because we created new settings, but they weren't added to the list of settings to be serialized, when they should have been. We should have a test that forces engineers introducing new settings to choose whether or not to add it to the exported list of settings.

[crisptrutski]

This has been broken up into multiple parts.

• Add mechanism for explicit import.
• Make all exported settings explicit in their definition.
• Make it impossible to add new settings without explicit :export? in definition.