feat: controll your tabs externally

[tavoli]

Hi, I'm amazed how we managed to do amazing things with so little svelte code and I love your lib.

I lovely make this

<script>
  let tabComponent
</script>

<Tabs bind:this={tabComponent}>
 ...
</Tabs>

<button on:click={() => tabComponent.nextTab()}>next tab</button>

You would appreciate my PR?

[rgwch]

It would be great having this possibility to switch tabs programatically. Or is there another way?