Enable better typescript support

[ArcticKeaton]

Currently the items do not use generics to keep the typings available. e.g.

<script lang="ts">
    let data = [ { id: 123, text: "item text" } ]
</script>
<VirtualScroll {data} let:data>
    <div>
        <!-- data is "any", no typings info -->
        {data.text}
    </div>
</VirtualScroll>

My proposal is to configure tighter typings for data, slot data and key:

data: T[]
key: keyof T
slot data: T