refactor: use $$restProps API

- add ref prop for applicable components (#196)
- add slot to Content Switcher `Switch` component (#183)
- remove fillArray, css utilities

src/components/FileUploader/FileUploader.Skeleton.svelte

@@ -1,15 +1,16 @@
 <script>
-  let className = undefined;
-  export { className as class };
-  export let style = undefined;
-
-  import { cx } from '../../lib';
-  import { ButtonSkeleton } from '../Button';
-  import SkeletonText from '../SkeletonText';
+  import { ButtonSkeleton } from "../Button";
+  import { SkeletonText } from "../SkeletonText";
 </script>
 
-<div on:click on:mouseover on:mouseenter on:mouseleave class={cx('--form-item', className)} {style}>
+<div
+  class:bx--form-item={true}
+  {...$$restProps}
+  on:click
+  on:mouseover
+  on:mouseenter
+  on:mouseleave>
   <SkeletonText heading width="100px" />
-  <SkeletonText width="225px" class={cx('--label-description')} />
+  <SkeletonText width="225px" class="bx--label-description" />
   <ButtonSkeleton />
 </div>