Support mutiple select in Combobox

### Prerequisites - [x] This feature already exists in [shadcn/ui](https://github.com/shadcn/ui) - if not, it won't be considered here so don't continue with your issue. ### Describe the feature The shadcn-svelte Combobox does not support multi-select. shadcn/ui shipped this in early 2026: https://ui.shadcn.com/docs/components/radix/combobox#multiple Multi-select Combobox shipped upstream in [shadcn-ui/ui#9022](https://github.com/shadcn-ui/ui/pull/9022) on December 12, 2025, and was published in the public docs on January 20, 2026 via [shadcn-ui/ui#9304](https://github.com/shadcn-ui/ui/pull/9304). The new primitive is built on the unified `radix-ui` / Base UI foundation and includes a `multiple` prop along with `ComboboxChips`, `ComboboxChip`, `ComboboxChipsInput`, and `ComboboxValue` parts. <img width="676" height="501" alt="Image" src="https://github.com/user-attachments/assets/4da68f4b-2507-4623-9d86-e56db4d948dd" />