Support TreeView item label templating

[brunnerh]

Currently the TreeView does not allow much customization in how the items are displayed.
It would be helpful if an optional slot is provided that is passed the item instance, e.g.

<TreeView {children}>
  <svelte:fragment slot="item" let:item>
     <span class:important={item.important}>{item.text}</span>
  </svelte:fragment>
</TreeView>

[theetrain]

This sounds appropriate to me, especially given the proposed state for slots in #1621.

[metonym]

Released in v0.82.0.

Docs.

<TreeView children="{children}" let:node>
  {node.id}
  {node.text}
  {node.expanded}
  {node.selected}
  {node.disabled}
  {node.leaf} // True if the node is a leaf (node does not have children)
</TreeView>

[brunnerh]

I think this is too restrictive, though.

In my example I access the source item as is, so additional properties can be used for styling or data display.

Maybe that should be split into two props, node and item where node contains any metadata and item is just the object as it has been passed in as part of children (without any modification to maintain the prototype). Ideally children would use a generic type parameter so type information can be propagated to the slot.