Enable Multiple mode for Combobox

[Uncle798]

Link to the Page

https://www.skeleton.dev/docs/integrations/popover/svelte/#combobox

Describe the Issue (screenshots encouraged!)

multiple doesn't actually exist as a prop.
defaultValue doesn't exist either

[endigo9740]

Extending this with a bit more context: The Zag.js primitive for Combobox does support a multiple feature, but it'll take a non-trivial amount of changes to the component to support this. To help ship the component sooner we skipped this feature. So I'm bumping this from a Task -> Feature (aka feature request) to better represent the ask here.