feat(component): add MultiSelect

Closes #19

src/components/ListBox/ListBoxMenu.svelte

@@ -1,7 +1,7 @@
 <script>
   let className = undefined;
   export { className as class };
-  export let id = Math.random(); // TODO: Use context to re-use same id per ListBox
+  export let id = Math.random();
   export let style = undefined;
 
   import { cx } from '../../lib';
@@ -9,6 +9,11 @@
   $: menuId = `menu-${id}`;
 </script>
 
-<div role="listbox" id={menuId} class={cx('--list-box__menu', className)} {style}>
+<div
+  role="listbox"
+  id={menuId}
+  aria-label={$$props['aria-label']}
+  class={cx('--list-box__menu', className)}
+  {style}>
   <slot />
 </div>