Removes all hardcoded overmap_connection_id references

[Procyonae]

Summary

None

Purpose of change

Hardcoded references bad
Allows mods/regions to have different connections for inter city vs intra city vs normal specials road connections, the former of which was impossible before while the latter required redefining every special to change it's connection
Is also useful for debugging city/inter city roads
Also fixes #72532 bc it's just a 1 line oversight and in the same file

Describe the solution

Adds a new region setting category for the connections with documentation explaining their uses and requirements
Replacing the intra city connections doesn't work perfectly because there's a few hardcoded references to the road oter so for example you can end up with two parallel adjacent roads, will look at making this more seamless in a future PR.

Describe alternatives you've considered

I were going to have them default in region_settings.h and make the category optional so it needn't be defined anywhere unless being overridden but that kind of defeats the purpose of removing the references

Testing

Used the test mod below to check both the new settings and overlay work correctly

[
  {
    "type": "MOD_INFO",
    "id": "testoverlay",
    "name": "testoverlay",
    "authors": [ "testoverlay" ],
    "maintainers": [ "testoverlay" ],
    "description": "testoverlay",
    "category": "rebalance",
    "dependencies": [ "dda" ]
  },
  {
    "type": "region_overlay",
    "regions": [ "all" ],
    "overmap_connection_settings": { "inter_city_road_connection": "z0_subway", "intra_city_road_connection": "z0_sewers" }
  },
  {
    "type": "overmap_connection",
    "id": "z0_sewers",
    "subtypes": [
      { "terrain": "sewer", "locations": [ "road", "z0_sewers" ], "basic_cost": 0, "flags": [ "ORTHOGONAL" ] },
      { "terrain": "sewer", "locations": [ "field" ], "basic_cost": 5 },
      { "terrain": "sewer", "locations": [ "forest" ], "basic_cost": 20 },
      { "terrain": "sewer", "locations": [ "swamp" ], "basic_cost": 40 },
      { "terrain": "sewer", "locations": [ "water" ], "basic_cost": 120 }
    ]
  },
  {
    "type": "overmap_connection",
    "id": "z0_subway",
    "subtypes": [
      { "terrain": "sewer", "locations": [ "z0_sewers" ], "basic_cost": 0, "flags": [ "ORTHOGONAL" ] },
      { "terrain": "subway", "locations": [ "field" ], "basic_cost": 5 },
      { "terrain": "subway", "locations": [ "forest" ], "basic_cost": 20 },
      { "terrain": "subway", "locations": [ "swamp" ], "basic_cost": 40 },
      { "terrain": "subway", "locations": [ "water" ], "basic_cost": 120 }
    ]
  },
  {
    "type": "overmap_connection",
    "id": "local_road",
    "subtypes": [
      { "terrain": "subway", "locations": [ "z0_subway" ], "basic_cost": 0, "flags": [ "ORTHOGONAL" ] },
      { "terrain": "sewer", "locations": [ "z0_sewers" ], "basic_cost": 0, "flags": [ "ORTHOGONAL" ] },
      { "terrain": "road", "locations": [ "road" ], "basic_cost": 0, "flags": [ "ORTHOGONAL" ] },
      { "terrain": "road", "locations": [ "field" ], "basic_cost": 5 },
      { "terrain": "road", "locations": [ "forest" ], "basic_cost": 20 },
      { "terrain": "road", "locations": [ "swamp" ], "basic_cost": 40 },
      { "terrain": "road_nesw_manhole", "locations": [  ] },
      { "terrain": "city_center", "locations": [ "field", "road" ] },
      { "terrain": "bridge", "locations": [ "water" ], "basic_cost": 120 },
      { "terrain": "bridgehead_ground", "locations": [ "water" ], "basic_cost": 120, "flags": [ "ORTHOGONAL" ] }
    ]
  },
  {
    "type": "overmap_location",
    "id": "z0_sewers",
    "terrains": [ "sewer", "city_center", "road_nesw_manhole" ]
  },
  {
    "type": "overmap_location",
    "id": "z0_subway",
    "terrains": [ "subway" ]
  }
]

Additional context

This could be used to yeet the SIDEWALK flag

[github-actions]

Auto-requesting reviews from non-collaborators: @I-am-Erk @Night-Pryanik