Added withProperties in BlockAlternative.java

Example: 

blockAlternative.withProperties("facing");

Return the facing of the block a empty string

src/main/java/net/minestom/server/instance/block/BlockAlternative.java

@@ -31,6 +31,16 @@ public class BlockAlternative {
         return map;
     }
     
+    public String withProperties(String key) {
+        for (String p : properties) {
+            String[] parts = p.split("=");
+            if (parts.length > 1)
+                if (parts[0].equals(key))
+                    return parts[1];
+        }
+        return "";
+    }
+
     @Override
     public String toString() {
         return "BlockAlternative{" +