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/NumberInput/NumberInput.Skeleton.svelte

@@ -1,15 +1,16 @@
 <script>
-  let className = undefined;
-  export { className as class };
   export let hideLabel = false;
-  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>
   {#if !hideLabel}
-    <span class={cx('--label', '--skeleton')} />
+    <span class:bx--label={true} class:bx--skeleton={true} />
   {/if}
-  <div class={cx('--number', '--skeleton')} />
+  <div class:bx--number={true} class:bx--skeleton={true} />
 </div>