feat(components): add Dropdown

Closes #22

- Revise ListBox ancillary components
- Pass item id to ListBoxMenuItem in MultiSelect, ComboBox

src/components/Dropdown/Dropdown.Skeleton.svelte

@@ -0,0 +1,20 @@
+<script>
+  let className = undefined;
+  export { className as class };
+  export let inline = false;
+  export let style = undefined;
+
+  import { cx } from '../../lib';
+</script>
+
+<div
+  on:click
+  on:mouseover
+  on:mouseenter
+  on:mouseleave
+  class={cx('--skeleton', '--dropdown-v2', '--list-box', '--form-item', inline && '--list-box--inline', className)}
+  {style}>
+  <div role="button" class={cx('--list-box__field')}>
+    <span class={cx('--list-box__label')} />
+  </div>
+</div>