[WorldGen] Mineshaft Overgrowth!

This modification overrides the vanilla structure set located at data/minecraft/worldgen/structure_set/mineshafts.json, dramatically increasing mineshaft generation frequency from the default value of roughly 0.004 to an extreme 0.50 using legacy_type_3 with spacing set to 1 and separation to 0, resulting in mineshafts becoming extraordinarily common, frequently overlapping and intersecting to create an almost endless subterranean labyrinth, while also intentionally pushing world generation to a heavier and more chaotic state that can slow generation and produce visually dense underground areas, making it strongly recommended to use only in a newly created world or by exploring entirely new chunks in order to properly observe the effect.