registerWorldFormat for worlds with embedded tilemaps

I'm attempting to write a map import/export handler for a platform game that has rooms of arbitrary size that can be moved around. In Tiled this translates to a world with multiple tilemaps. Reading the Tiled documentation it doesn't look like worlds can have the tilemaps embedded.

The problem is that this game stores this data in one file per "world", and I can't see a way to write a registerWorldFormat that would then generate the world and all of its tilemaps in a single data structure (and be able to save that entire structure back to a single file).

It would be great if tilemaps could be embedded in the world data, and if you could then register a file handler instead of being restricted to .world JSON files.

Although I could just load the entire world into one great big tilemap, it loses the ability to define room sizes.

Start by reviewing Tiled's world and tilemap format documentation and the existing registerWorldFormat behavior. Done means a world can contain multiple embedded tilemaps, preserve room sizes and positions, and be loaded from and saved to the requested single-file format.