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/ComposedModal/ModalBody.svelte

@@ -1,20 +1,16 @@
 <script>
-  let className = undefined;
-  export { className as class };
   export let hasForm = false;
   export let hasScrollingContent = false;
-  export let style = undefined;
-
-  import { cx } from '../../lib';
 </script>
 
 <div
   tabindex={hasScrollingContent ? '0' : undefined}
   role={hasScrollingContent ? 'region' : undefined}
-  class={cx('--modal-content', hasForm && '--modal-content--with-form', className)}
-  {style}>
+  class:bx--modal-content={true}
+  class:bx--modal-content--with-form={hasForm}
+  {...$$restProps}>
   <slot />
 </div>
 {#if hasScrollingContent}
-  <div class={cx('--modal-content--overflow-indicator')} />
+  <div class:bx--modal-content--overflow-indicator={true} />
 {/if}