Port multiselect from cookbook project

[Wolfr]

In https://github.com/Wolfr/svelte-mycookbook a multi select was built.

Based on REPL: https://svelte.dev/repl/c7094fb1004b440482d2a88f4d1d7ef5?version=3.14.0

Todo: port to this main project.

Demo at https://svelte-mycookbook-wolfr.vercel.app/