Make this library more Svelte idiomatic

[enyo]

I have the feeling, that the positioning logic in this library is a bit shaky. That's not a criticism, because it's very hard to do, but I have the feeling that Svelte could do a lot of the heavy lifting there.

Instead of calculating the heights, and animating between them programmatically, you could just use an in: and out: animation, to automatically transition between the different positions the toasts are displayed at. This way, the layout of the toasts could be done by the CSS layout engine (by simply placing them in a flexbox for example) and the animation between these states would be automatic.

Would you be interested in attacking such a change?