Add UNKNOWN portal type, make generatePortal return a boolean (#2386)

Author: dualspiral

src/main/java/org/spongepowered/api/world/portal/PortalType.java

@@ -49,8 +49,9 @@ public interface PortalType extends DefaultedRegistryValue {
      *
      * @param location The location
      * @param axis The axis
+     * @return true if a portal could be generated
      */
-    void generatePortal(ServerLocation location, Axis axis);
+    boolean generatePortal(ServerLocation location, Axis axis);
 
     /**
      * Finds a {@link Portal} from a {@link ServerLocation location}.

src/main/java/org/spongepowered/api/world/portal/PortalTypes.java

@@ -40,9 +40,11 @@ public final class PortalTypes {
 
     // SORTFIELDS:ON
 
+    public static final DefaultedRegistryReference<PortalType> END = PortalTypes.key(ResourceKey.sponge("end"));
+
     public static final DefaultedRegistryReference<PortalType> NETHER = PortalTypes.key(ResourceKey.sponge("nether"));
 
-    public static final DefaultedRegistryReference<PortalType> END = PortalTypes.key(ResourceKey.sponge("end"));
+    public static final DefaultedRegistryReference<PortalType> UNKNOWN = PortalTypes.key(ResourceKey.sponge("unknown"));
 
     // SORTFIELDS:OFF