feat(toolkit): add MultiSelect component for shadcn kit

[KylianCodes]

Adds a multi-select component that displays selected options as removable badges, backed by a Stimulus controller and a hidden select multiple for standard form submission.

Relates to #3233

Q	A
Bug fix?	no
New feature?	yes
Deprecations?	no
Documentation?	no
Issues	#3233
License	MIT

Adds a MultiSelect component to the shadcn kit. Selected options are displayed as removable badges inside the trigger. A hidden <select multiple> handles standard Symfony form submission.

Usage

<twig:MultiSelect name="fruits" placeholder="Select fruits...">
    <twig:MultiSelect:Option value="apple" label="Apple" />
    <twig:MultiSelect:Option value="banana" label="Banana" />
</twig:MultiSelect>