fix(combobox): value should call itemToString with filtered item (#1411)

Fixes #1405

When using the `shouldFilterItem` prop, the `ComboBox` does not display the custom label set with `itemToString` in the input after a selection.

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

@@ -0,0 +1,34 @@
+<script>
+  import { ComboBox } from "carbon-components-svelte";
+
+  const translation = {
+    Slack: 'Custom label for "Slack"',
+    Email: 'Custom label for "Email"',
+    Fax: 'Custom label for "Fax"',
+  };
+
+  function itemToString(item) {
+    return translation[item.key];
+  }
+
+  function shouldFilterItem(item, value) {
+    if (!value) return true;
+    const comparison = value.toLowerCase();
+    return (
+      item.key.toLowerCase().includes(comparison) ||
+      itemToString(item).toLowerCase().includes(comparison)
+    );
+  }
+</script>
+
+<ComboBox
+  titleText="Contact"
+  placeholder="Select contact method"
+  items="{[
+    { id: '0', key: 'Slack' },
+    { id: '1', key: 'Email' },
+    { id: '2', key: 'Fax' },
+  ]}"
+  shouldFilterItem="{shouldFilterItem}"
+  itemToString="{itemToString}"
+/>