Use an enum to specify overmap tile see cost instead of a number

[ehughsbaird]

Summary

Infrastructure "Specify see_cost for overmap terrain with an enum instead of an integer"

Purpose of change

5e1c99a fixes an accident made in e44c61f

Following #75236, overmap vision needs adjustment. Using an enum makes it easier to tweak and tune the system, as the categories correspond to physical characteristics of the overmap terrains and what numeric values they correspond to can be swiftly tweaked.

Describe the solution

Replace all instances of see_cost": [0-9] with an enum. Add the enum and handling for it, and update documentation.

name	role
"all_clear"	This tile has no or minimal horizontal obstacles and can be seen down through
"none"	This tile has no or minimal horizontal obstacles - most flat terrain
"low"	This tile has low horizontal obstacles or few higher obstacles
"medium"	This tile has medium horizontal obstacles
"spaced_high"	This tile has high obstacles, but they are spaced and have several gaps - a forest
"high"	This tile has high obstacles, but still allows some sight around it - most buildings
"full_high"	This tile has high obstacles, and effectively cannot be seen through - multi-tile buildings
"opaque"	This tile cannot be seen through under any circumstance

Describe alternatives you've considered

More or different enum values could be used.

Testing

The game loads with all mods enabled, and overmap vision works as expected.

[ehughsbaird]

The new push rebases the PR and adds a see_cost to the terrain from #75381 so the game won't stop loading when this PR is merged.