feat(skeleton-placeholder): add parts to allow for external styling (#19972)

Author: annawen1

packages/web-components/src/components/ai-skeleton/ai-skeleton-placeholder.ts

@@ -20,6 +20,7 @@ import { carbonElement as customElement } from '../../globals/decorators/carbon-
 class CDSAISkeletonPlaceholder extends LitElement {
   render() {
     return html`<cds-skeleton-placeholder
+      exportparts="placeholder:skeleton-placeholder"
       optional-classes="${prefix}--skeleton__placeholder--ai"></cds-skeleton-placeholder>`;
   }
 

packages/web-components/src/components/skeleton-placeholder/skeleton-placeholder.ts

@@ -39,7 +39,7 @@ class CDSSkeletonPlaceholder extends LitElement {
     }
     const classes = classMap(defaultClasses);
 
-    return html` <div class="${classes}"></div> `;
+    return html` <div part="placeholder" class="${classes}"></div> `;
   }
 
   static styles = styles;