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/FormItem/FormItem.svelte

@@ -1,11 +1,9 @@
-<script>
-  let className = undefined;
-  export { className as class };
-  export let style = undefined;
-
-  import { cx } from '../../lib';
-</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>
   <slot />
 </div>