Random cleanups to users of Level.OVERWORLD and reduce minecraft_server usage (#1697)

- Use `overworld()` instead of `getLevel(Level.OVERWORLD)` when convenient
- Remove now unnecessary `// OW` comments, from when it had no mappings
- Make spawn chunk size and light batches not depend on global server instance
- Merge all spawn chunk size logic into SpawnChunks instead of methods in the validator
- Remove unneeded `withWorld(OVERWORLD)` with just-created server `CommandSource`s
- Use enhanced for in light batch application

src/main/java/carpet/mixins/ServerPlayer_scarpetEventMixin.java

@@ -110,7 +110,7 @@ public abstract class ServerPlayer_scarpetEventMixin extends Player implements S
         {
             ServerPlayer player = (ServerPlayer) (Object)this;
             Vec3 to = null;
-            if (!wonGame || previousDimension != Level.END || destination.dimension() != Level.OVERWORLD) // end ow
+            if (!wonGame || previousDimension != Level.END || destination.dimension() != Level.OVERWORLD)
             {
                 to = position();
             }