Add option to not re-focus ComboBox when invoking clear() (#1000)

* feat(combo-box): add option to clear accessor to not re-focus input

Closes #994

* yarn build:lib

* test(combo-box): validate clear accessor params

* docs(combo-box): add clear without focus to "Clear selection" example

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

@@ -0,0 +1,20 @@
+<script>
+  import { ComboBox, Button } from "carbon-components-svelte";
+
+  let ref;
+</script>
+
+<ComboBox
+  titleText="Contact"
+  placeholder="Select contact method"
+  selectedIndex="{1}"
+  bind:this="{ref}"
+  items="{[
+    { id: '0', text: 'Slack' },
+    { id: '1', text: 'Email' },
+    { id: '2', text: 'Fax' },
+  ]}"
+/>
+<br />
+<Button on:click="{ref.clear}">Clear</Button>
+<Button on:click="{() => ref.clear({ focus: false })}">Clear (no focus)</Button>