Configurable sub-level assembly height

Adds 2 config values, implementing suggestions 2 & 3 from #938:
- SUB_LEVEL_START_POS: Determines the approach to use for determining the height (Y value) of the first block in a newly assembled sub-level (FIXED, DYNAMIC, MIDPOINT)
  - FIXED: Uses the height specified by SUB_LEVEL_FIXED_START_HEIGHT (Essentially the current approach)
  - DYNAMIC: Uses the world height of the position the block is in (Spawn a sub-level at Y=73, the block is placed at Y=73 in the sub-level)
  - MIDPOINT: Calculates the midpoint between the min and max build limits, then uses that as the height
- SUB_LEVEL_FIXED_START_HEIGHT 
  - If SUB_LEVEL_START_POS is set to FIXED, then this is the height (Default value of 128 matches the current approach)

Author: tatoyoda600

common/src/main/java/dev/ryanhcode/sable/SableConfig.java

@@ -3,6 +3,7 @@
 import net.neoforged.neoforge.common.ModConfigSpec;
 
 public final class SableConfig {
+    public enum SubLevelStartPos { FIXED, DYNAMIC, MIDPOINT }
 
     public static final ModConfigSpec SPEC;
 
@@ -19,6 +20,8 @@ public final class SableConfig {
     public static final ModConfigSpec.BooleanValue ATTEMPT_UDP_NETWORKING;
     public static final ModConfigSpec.BooleanValue VERBOSE_SERIALIZATION_LOGGING;
     public static final ModConfigSpec.BooleanValue SUB_LEVEL_SAVING_LOG_MESSAGE;
+    public static final ModConfigSpec.EnumValue<SubLevelStartPos> SUB_LEVEL_START_POS;
+    public static final ModConfigSpec.IntValue SUB_LEVEL_FIXED_START_HEIGHT;
 
     static {
         final ModConfigSpec.Builder builder = new ModConfigSpec.Builder();
@@ -64,6 +67,12 @@ public final class SableConfig {
         VERBOSE_SERIALIZATION_LOGGING = builder
                 .comment("If Sable should use verbose logging for its serialization system and the holding chunk-map. Not recommended- for debugging purposes only.")
                 .define("verbose_serialization_logging", false);
+        SUB_LEVEL_START_POS = builder
+                .comment("When a sub-level is created, how to determine the height within the sub-level that the first block should be placed at.")
+                .defineEnum("sub_level_start_pos", SubLevelStartPos.FIXED);
+        SUB_LEVEL_FIXED_START_HEIGHT = builder
+                .comment("If 'sub_level_start_pos' is set to 'FIXED', the fixed height to use.")
+                .defineInRange("sub_level_fixed_start_height", 128, Integer.MIN_VALUE, Integer.MAX_VALUE);
 
         SPEC = builder.build();
     }

common/src/main/java/dev/ryanhcode/sable/api/SubLevelAssemblyHelper.java

@@ -1,6 +1,7 @@
 package dev.ryanhcode.sable.api;
 
 import dev.ryanhcode.sable.Sable;
+import dev.ryanhcode.sable.SableConfig;
 import dev.ryanhcode.sable.api.block.BlockSubLevelAssemblyListener;
 import dev.ryanhcode.sable.api.physics.PhysicsPipeline;
 import dev.ryanhcode.sable.api.physics.handle.RigidBodyHandle;
@@ -84,7 +85,14 @@ public static ServerSubLevel assembleBlocks(final ServerLevel level, final Block
         final LevelPlot plot = subLevel.getPlot();
         plot.newEmptyChunk(plot.getCenterChunk());
 
-        final BlockPos plotAnchor = plot.getCenterBlock();
+        Integer yPos = 128;
+        switch (SableConfig.SUB_LEVEL_START_POS.get()) {
+                case FIXED -> yPos = SableConfig.SUB_LEVEL_FIXED_START_HEIGHT.getAsInt();
+                case DYNAMIC -> yPos = anchor.getY();
+                case MIDPOINT -> yPos = (level.getMinBuildHeight() + level.getMaxBuildHeight()) >> 1;
+        }
+
+        final BlockPos plotAnchor = plot.getCenterBlock().atY(yPos);
         final SubLevelAssemblyHelper.AssemblyTransform transform = new SubLevelAssemblyHelper.AssemblyTransform(anchor, plotAnchor, 0, Rotation.NONE, level);
         SubLevelAssemblyHelper.moveOtherStuff(level, transform, blocks, bounds);
         SubLevelAssemblyHelper.moveBlocks(level, transform, blocks);