Allow Add-Ons to rewrite or overwrite default biome and terrain generation

Being able to completely restructure world generation from top to bottom would be amazing for add-ons. Things like custom noise maps, world variables, land/sea tile generation, vanilla biome spawning requirements, structure spawn rates, all of it. This would allow marketplace creators to do things like bring back and re-tune continental generation from old Minecraft, make biomes larger or make similar biomes cluster more, allow for more involved and thematic transitionary biomes, improve the spawning of custom structures, make more interesting custom biomes and more. This would be great for more careful and involved creators who want to cultivate a specific experience with their add-on and for purely capitalistic creators who churn out whatever gets the most buys, so everyone wins if Mojang can get this working. This might cause problems with world corruption depending on if creators can only overwrite the default world generation instead of disabling and replacing it, but adding an extra warning for add-ons with advanced world generation should be enough of a safeguard to keep people from messing up their worlds too much. Multiple add-ons using advanced world generation might be a problem but it should already be mostly solved by the add-on priority system worlds have. Tools to allow developers to make their advanced world generation compatible with other creators custom biomes would be appreciated as well but that's a nice to have and not a need to have.