Configured Feature Saplings

This mod fixes the bug that prevents datapacks from customizing the configured features that grow from saplings and mushrooms. (https://bugs.mojang.com/browse/MC-251744)

In short, it allows datapacks to edit features in the `worldgen/configured_features` folder and have those changes apply when that feature is grown from a sapling or mushroom. If one were to try the same thing in vanilla, the customized worldgen feature would only be referenced when generating it in new chunks, while growing it from a sapling or mushroom would yield the hardcoded vanilla feature.