feat: implement custom rendering

[chaoran-chen]

I'm trying to implement custom rendering using Svelte's slots. The implementation of this PR allows the library user to define a global custom rendering which will be applied to all items:

<ImageGallery>
  <div slot="render" let:item>
    <!-- ... -->
  </div>
</ImageGallery>

The open issues for which I don't know a solution are:

1. How can we implement per-item custom rendering? It seems that the name of the slots may not be dynamic: i.e., I think (and I can be wrong!) that we can't dynamically create a slot for each item.
2. npm run check is throwing an error that I don't understand:

ImageGallery.svelte:542:19
Error: Property 'default' does not exist on type '{ render: { slot: string; item: Partial<{ bulletClass: string; description: string; original: string; originalHeight: number; originalWidth: number; loading: LoadingStrategy; thumbnailHeight: number; ... 13 more ...; sizes: string; }>; }; }'. (ts)

3. The code is quite ugly. In ImageGallery, we now use <SlideWrapper> four times and in SlideWrapper, we have <Slide> four times..

I added a use custom rendering checkbox to the demo page for trying it out.