feat(combo-box): make ComboBox slottable (#1181)

Closes #1176

docs/src/pages/framed/ComboBox/ComboBoxSlot.svelte

@@ -0,0 +1,29 @@
+<script>
+  import { ComboBox } from "carbon-components-svelte";
+</script>
+
+<ComboBox
+  titleText="Contact"
+  placeholder="Select contact method"
+  items="{[
+    { id: '0', text: 'Slack' },
+    { id: '1', text: 'Email' },
+    { id: '2', text: 'Fax' },
+  ]}"
+  let:item
+  let:index
+>
+  <div>
+    <strong>{item.text}</strong>
+  </div>
+  <div>
+    id: {item.id} - index:
+    {index}
+  </div>
+</ComboBox>
+
+<style>
+  :global(.bx--list-box__menu-item, .bx--list-box__menu-item__option) {
+    height: auto;
+  }
+</style>