feat(components): add Dropdown

Closes #22

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

src/components/Dropdown/Dropdown.Story.svelte

@@ -0,0 +1,49 @@
+<script>
+  export let story = undefined;
+
+  import Layout from '../../internal/ui/Layout.svelte';
+  import Button from '../Button';
+  import Dropdown from './Dropdown.svelte';
+  import DropdownSkeleton from './Dropdown.Skeleton.svelte';
+
+  let items = [
+    { id: 'option-0', text: 'Option 1' },
+    { id: 'option-1', text: 'Option 2' },
+    { id: 'option-2', text: 'Option 3' },
+    { id: 'option-3', text: 'Option 4' }
+  ];
+
+  let selectedIndex = -1;
+</script>
+
+<Layout>
+
+  {#if story === 'skeleton'}
+    <div style="width: 300px">
+      <DropdownSkeleton />
+      &nbsp;
+      <DropdownSkeleton inline />
+    </div>
+  {:else}
+    <p>Currently, this component does not support items as slots.</p>
+    <p>
+      <code>items</code>
+      must be an array of objects; mandatory fields are `id` and `text`.
+    </p>
+    <pre style="margin-top: 1rem;">
+      <code>{'items = Array<{ id: string; text: string; }>'}</code>
+    </pre>
+    <div style="margin-top: 2rem; margin-bottom: 2rem;">
+      <Button
+        size="small"
+        on:click={() => {
+          selectedIndex = selectedIndex > -1 ? -1 : 1;
+        }}>
+        {selectedIndex > -1 ? 'Clear selected item' : "Set item to 'Option 2'"}
+      </Button>
+    </div>
+    <div style="width: 300px">
+      <Dropdown {...$$props} bind:selectedIndex {items} />
+    </div>
+  {/if}
+</Layout>