SPIGOT-5877: Add scaffolding for custom dimensions and biomes

By: Martoph <sager1018@gmail.com>

paper-api/src/main/java/org/bukkit/World.java

@@ -2430,7 +2430,11 @@ public interface World extends PluginMessageRecipient, Metadatable {
         /**
          * Represents the "end" map
          */
-        THE_END(1);
+        THE_END(1),
+        /**
+         * Represents a custom dimension
+         */
+        CUSTOM(-999);
 
         private final int id;
         private static final Map<Integer, Environment> lookup = new HashMap<Integer, Environment>();

paper-api/src/main/java/org/bukkit/block/Biome.java

@@ -87,7 +87,11 @@ public enum Biome implements Keyed {
     SOUL_SAND_VALLEY,
     CRIMSON_FOREST,
     WARPED_FOREST,
-    BASALT_DELTAS;
+    BASALT_DELTAS,
+    /**
+     * Represents a custom Biome
+     */
+    CUSTOM;
 
     private final NamespacedKey key;