Terrain Adaptation Template Pool Element for Jigsaw structures

Currently, Terrain Adaptation (beard_thin, beard_box, bury, and encapsulate) doesn't work for all custom structures: they don't account for drastic changes in the y-axis, there isn't an option to completely expose the structure, and there is no way to finely control where the ground or air is placed (especially on a room-by-room basis).

There are work-arounds for terrain adaptation for custom jigsaw structures (placing all-air or all-ground placed features, adding all-air or all-ground 'padding' jigsaw structures), but they have their own issues--they cant distinguish between the structure and its surrounding terrain, can fail to generate due to self-intersections, and cannot be made to respect the biome. Additionally, these work-arounds do not behave well with worldgen placed features or Surface Rules.

A new element type for Template Pools could be added that modifies the final terrain density (like how the Terrain Adaptation for structures is already able to). It should be configurable in size, possibly shape, and the effect on the final density (positive for 'solid ground', and negative for 'open space'). As it would be a template pool element, it would be able to be placed by jigsaws in jigsaw structures, like how Feature Pool Elements can place features.

This would allow Jigsaw structures to more finely tune their terrain adaptation, and make it work with all kinds of structures, beyond what the current options of beard_thin, beard_box, bury, and encapsulate can accomplish.