fix: add part to slider

source/components/layout/slider.mjs

@@ -745,7 +745,7 @@ function getTemplate() {
             <div data-monster-role="slider" part="slides">
                 <slot></slot>
             </div>
-            <div data-monster-role="thumbnails"></div>
+            <div data-monster-role="thumbnails" part="thumbnails"></div>
             <div class="next" data-monster-role="next" part="next" part="next">
                 <slot name="next"></slot>
             </div>