Further expand regional terrains and work them into some map palettes

[I-am-Erk]

Summary

SUMMARY: Infrastructure "Further expands regional pseudoterrains and works them into some map palettes"

Purpose of change

Regional terrains were added in #35627 and expanded in #35669. This powerful new feature needs some further expansion to be fully useful, eg. in the addition of new biome types, or for the rapid adjustment of the appearance of terrain types over all maps.

Describe the solution

Added t_region_groundcover_forest and _swamp
Added f_region_water_plant
Debated adding t_region_tree_swamp. Might still do it but I'm trying to think of a more elegant solution than that.

Converted occurrence of trees and grass and shrubs and flowers to regional equivalents in several map palettes, and will continue to do so. This adds some variance to maps, and standardizes the occurrence of long grass and stuff like that. Later, it will mean that a biome can specify different appearances, such as sand and dry earth.

Describe alternatives you've considered

There are infinite ways the terrains could be broken down.

There's some argument for keeping more specific appearances of things like t_tree, where one might want specifically an oak tree, but I don't think there are many of those and overall it would be better to ask for a shade tree and allow some random variation I think.

Testing

When it's done.