Detailed description of the feature

Add option for the mirrored Overworld to have it's own Terrain generation + it's own set of Structures by making specific Mods and Datapacks believe that's the Overworld, while the main one doesn't get affected by this list of Mods/Datapacks.
Of course, any item and mechanic they add to the game still works outside of the Mirror World, but they would only modify that landscape, and would also only spawn mobs there if specified.

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• I have checked that this is not a duplicate feature request and the feature does not yet exist in the latest version