feat: update MultiSelect.svelte for Svelte5 compatibility

[dedo1911]

Closes #281

Summary of major changes

1. fixes Svelte 5 compatibility issues

Todos

1. Change events to new Svelte 5 format

[janosh]

ideally, i'd prefer to update package deps and unit tests to Svelte 5 in one go. that requires an update to mdsvexamples and fixing how unit tests create component instances

const select = new MultiSelect({
  target: document.body,
  props: { options },
})

Attempted to instantiate src/lib/MultiSelect.svelte with new MultiSelect, which is no longer valid in Svelte 5. If this component is not under your control, set the compatibility.componentApi compiler option to 4 to keep it working. See https://svelte-5-preview.vercel.app/docs/breaking-changes#components-are-no-longer-classes for more information

[janosh]

svelte 5 compatibility is time sensitive so merging as is since upstream blockers could take months to resolve or might even require a docs rework since mdsvexamples looks to be unmaintained

[dedo1911]

In fact my idea was not to update deps to Svelte 5 already, but just make it work for people who can't wait, solving the build issues. I'm already using it in production and works just fine.
That was the reason of the smallest fix possible. This PR was not yet intended to provide full support to Svelte 5 features.
We'll dig into what's needed for the rework in another pr.

[janosh]

that would be great, looking forward to that PR! 👍

could you give v11.0.0-rc.1 a try and let me know how it goes?

[dedo1911]

could you give v11.0.0-rc.1 a try and let me know how it goes?

Looks good to me!