Default slot as named slot fallback with let directive triggers ReferenceError

[janosh]

I think I might be running into a bug with slots but I can't reliably reproduce outside SvelteKit.

The change in question that seems to cause the error is adding a default slot as a fallback inside a named slot (see changes). The error seems to stem from a let: directive to pass an option prop to the default prop.

ReferenceError: option is not defined
    at eval (/home/projects/github-qcxbgh/src/lib/MultiSelect.svelte:556:31)
    at Object.$$render (/home/projects/github-qcxbgh/node_modules/svelte/internal/index.mjs:2040:22)

@dummdidumm I know it's a lot to ask but maybe you could take a quick lock at this stackblitz. Not sure how to further debug.