code: public boolean isExtensionLoaded(String classname) {
          for (Extension extension : getLoadedExtensions()) {
              if (classname.equals(extension.getClass().getCanonicalName())) {
                  return true;
              }
          }
          return false;
      }
docs: Tells whether or not the extension with the given {@code classname} is loaded.
  
  @param classname the classname of the extension
  @return {@code true} if the extension is loaded, {@code false} otherwise
  @since 2.4.0