Add in missing terrain properties

[RobProductions]

Need to control the pixel error, base map dist, cast shadows, reflection probes, contribute GI setting, tree prototype layers setting (maybe goes in terrain object properties), and bake light probes for trees setting

[RobProductions]

Some properties like contribute GI are more complicated than expected and will be tackled in a future update. Other settings that are left are for trees (tree distance, billboard start, fade length, and max mesh trees which may only work for Unity Trees or prefabs with billboards in the LOD group) and for detail objects (detail distance, detail density, grass speed/size/bending/tint). Would also be nice to set the material, but there should be a way to easily add the TerrainLit default material back in, and I'm not sure how to grab it from code. This could be an enum between default material and custom material with an object field popping up for you to enter a custom terrain material.