New gamerule: liquidHardens

liquidHardens would be true by default. When set to false, it makes flowing lava destroy any water that it goes through and flowing water destroy flowing lava it goes through. If water touches a lava source, that would make the lava into obsidian. This prevents lava casting, as the flowing water would just destroy the flowing lava and vice Versa, instead of transforming anything into stone. You’d need to place a bunch of individual lava blocks to try and turn them all into obsidian, and at that point, you’d be better off placing blocks by hand.