Add Movable Amethyst Rule (#894)

* Add Pushable Amethyst Rule

As discussed in #79 of ilmangos peaceful challenge. He said he'd do it, I figured I could beat him to the punch :)
https://youtu.be/0-fAKpr_J8k?t=1076

* Name change - movableAmethyst

Co-authored-by: hugeblank <hugeblank@gmail.com>

src/main/java/carpet/mixins/BuddingAmethystBlock_getPistonBehaviorMixin.java

@@ -0,0 +1,18 @@
+package carpet.mixins;
+
+import carpet.CarpetSettings;
+import net.minecraft.block.BlockState;
+import net.minecraft.block.BuddingAmethystBlock;
+import net.minecraft.block.piston.PistonBehavior;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Inject;
+import org.spongepowered.asm.mixin.injection.callback.CallbackInfoReturnable;
+
+@Mixin(BuddingAmethystBlock.class)
+public class BuddingAmethystBlock_getPistonBehaviorMixin {
+    @Inject(at = @At("HEAD"), method = "getPistonBehavior(Lnet/minecraft/block/BlockState;)Lnet/minecraft/block/piston/PistonBehavior;", cancellable = true)
+    void getPistonBehavior(BlockState state, CallbackInfoReturnable<PistonBehavior> cir) {
+        if (CarpetSettings.movableAmethyst) cir.setReturnValue(PistonBehavior.NORMAL);
+    }
+}