feat: add usePlaceholder

src/composition/use-placeholder.js

@@ -0,0 +1,14 @@
+import { useHeight } from './use-rect';
+
+export function usePlaceholder(contentRef, bem) {
+  const height = useHeight(contentRef);
+
+  return (renderContent) => (
+    <div
+      class={bem('placeholder')}
+      style={{ height: height.value ? `${height.value}px` : null }}
+    >
+      {renderContent()}
+    </div>
+  );
+}