Fix lmoe bunker stairs misalignment and other minor issues

[BalthazarArgall]

Summary

None

Purpose of change

1. The lmoe bunker stairs are misaligned
2. Insane lmoe file organization, needless mapgen definitions duplicates

Describe the solution

• Remove the old bunker exit mapgens, replace them with a much smaller mapgen nest, spawned at the surface from the bunker mapgen itself. To accomodate this I had to slightly modify one of the layouts and give the nests a precise rotation since some of the stairs were located very close to the map borders.
• Remove all the map duplicates. It was an insane incomprehensible mess, I think the intent was to have looted/unlooted maps as well as populated (with monsters)/unpopulated maps but nowadays this can be easily and cleanly replicated with just one definition per layout. Technically I'm using a palette distribution to make 25% of the bunkers looted (using a null palette), and I'm using an overmap_terrain "spawns" field to spawn between 2 to 6 zombies in 50% of the bunkers.
• The bunker exit roofs have also been mapgen nestified while keeping the possible solar panel spawns.
• The special prepper quest bunker has had the same treatment but due to me being too dum I had to keep it it's own mapgen/overmap_terrain etc.
• Clean up the bunker exit from the random hard placed trees and bushes, now it uses the regional terrain, no more spot of trees in the middle of a naked field.

Describe alternatives you've considered

• Wasted a few hours trying to rework the prepper quest map thing so it could be any of the other layout but I couldn't do it.
• Don't null the loot palettes, I'm not sure we care to represent looted bunkers and if we do maybe it should be a bit more elegant than simply nulling the loot there. If pp generators can be made to remove loot we may be able to do it through that system in the future.
• Make each layout it's own mapgen id, this would require adding 3 more special definitions and I don't see why we would need to differentiate them at the moment.
• Remove the now unused terrain definitions from the palettes. Unneeded, plus maybe some other map somewhere uses it.

Testing

Spawned plenty of bunkers, looks fine to me. Admittedly not a lot of testing on the prepper quest logic part specifically but it should work.

Additional context

Fixes #86842.

[github-actions]

Auto-requesting reviews from non-collaborators: @thaelina

[GuardianDll]

Error: Mods-(~[slow] ~[.],starting_items)=> JSON contains circular dependency, this object is discarded

    "vision_levels": "unusual_structure",
    "travel_cost_type": "roof"
  },
   ▲▲▲
  {
    "type": "overmap_terrain",


Mods-(~[slow] ~[.],starting_items)=> 17:28:21.844 ERROR : src/flexbuffer_json.cpp:332 [void JsonObject::error_skipped_members(const std::vector<size_t> &) const] (json-error)
Error: Mods-(~[slow] ~[.],starting_items)=> Unread data.  Invalid or misplaced field name "copy-from" in JSON data

    "type": "overmap_terrain",
    "id": [ "last_mage_on_earth_shelter" ],
    "copy-from": "lmoe_under_empty"
               ▲▲▲
  },

[LeSHi4]

Greetings! I find little bug :)

[github-actions]

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