`CMultiOptionButton` `onClick` type is wrong

The type of the `CMultiOptionButton` is probably wrong:
```ts
  /**
   * Emitted when the user clicks on this button.
   */
  onClick?: () => {};
```

Should probably be:
```ts
  /**
   * Emitted when the user clicks on this button.
   */
  onClick?: () => void;
```