SlTree custom expand icons are not visible

[bikashghimire]

Describe the bug

When using custom expand and collapse icons, the icons are invisible in the UI. Clicking them does work but nothing is shown.

To Reproduce

Steps to reproduce the behavior:

<SlTreeItem>
  Deciduous
  <SlTreeItem>Birch</SlTreeItem>
  <SlTreeItem>
    Maple
    <SlTreeItem>Field maple</SlTreeItem>
    <SlTreeItem>Red maple</SlTreeItem>
    <SlTreeItem>Sugar maple</SlTreeItem>
  </SlTreeItem>
  <SlTreeItem>Oak</SlTreeItem>
</SlTreeItem>

<SlTreeItem>
  Coniferous
  <SlTreeItem>Cedar</SlTreeItem>
  <SlTreeItem>Pine</SlTreeItem>
  <SlTreeItem>Spruce</SlTreeItem>
</SlTreeItem>

<SlTreeItem>
  Non-trees
  <SlTreeItem>Bamboo</SlTreeItem>
  <SlTreeItem>Cactus</SlTreeItem>
  <SlTreeItem>Fern</SlTreeItem>
</SlTreeItem>

[akasht2509]

May be due to slot issue

<sl-tree>
  <sl-tree-item>
    <span slot="expand-icon">
      <sl-icon name="plusIcon"></sl-icon>
    </span>
    item
  </sl-tree-item>
</sl-tree>

https://shoelace.style/getting-started/usage#slots for more details