feat: add TypeScript definitions for itemLabelGenerator

- Add itemLabelGenerator type definition to SelectBaseMixinClass
- Add itemLabelGenerator type definition to ComboBoxItemsMixinClass
- MultiSelectComboBoxMixinClass inherits the type automatically via ComboBoxItemsMixinClass

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: Artur-

packages/combo-box/src/vaadin-combo-box-items-mixin.d.ts

@@ -50,4 +50,11 @@ export declare class ComboBoxItemsMixinClass<TItem> {
    * @attr {string} item-value-path
    */
   itemValuePath: string;
+
+  /**
+   * Function that is used to generate the label for each item.
+   * Receives one argument:
+   * - `item` The item to generate the label for.
+   */
+  itemLabelGenerator: ((item: TItem) => string) | undefined;
 }

packages/select/src/vaadin-select-base-mixin.d.ts

@@ -107,6 +107,13 @@ export declare class SelectBaseMixinClass {
    */
   noVerticalOverlap: boolean;
 
+  /**
+   * Function that is used to generate the label for each item.
+   * Receives one argument:
+   * - `item` The item to generate the label for.
+   */
+  itemLabelGenerator: ((item: SelectItem) => string) | undefined;
+
   /**
    * Requests an update for the content of the select.
    * While performing the update, it invokes the renderer passed in the `renderer` property.