added encode_nbt, added 'nbt' and 'nbt_merge' to modify entity API.

src/main/java/carpet/script/value/GlobalValue.java

@@ -1,6 +1,7 @@
 package carpet.script.value;
 
 import carpet.script.exception.InternalExpressionException;
+import net.minecraft.nbt.Tag;
 
 public class GlobalValue extends Value
 {
@@ -28,4 +29,10 @@ public class GlobalValue extends Value
     {
         throw new UnsupportedOperationException("Global value cannot be used as a map key");
     }
+
+    @Override
+    public Tag toTag(boolean force)
+    {
+        throw new UnsupportedOperationException("Global value cannot be serialized to the tag");
+    }
 }